Lucene migration: Format version is not supported

Nobody has claimed this yet.

I tried to move from a JanusGraph 0.1.1 Graph of the Gods graph with BerkeleyJE and Lucene over to JanusGraph 0.2.0-SNAPSHOT. Ran into the exception below:

```
gremlin> g.E().has('place', geoWithin( Geoshape.circle(37.6, 23.8, 20) )).values('place')
Could not process individual retrieval call
Type ':help' or ':h' for help.
Display stack trace? [yN]y
org.janusgraph.core.JanusGraphException: Could not process individual retrieval call
at org.janusgraph.graphdb.query.QueryUtil.processIntersectingRetrievals(QueryUtil.java:257)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6.execute(StandardJanusGraphTx.java:1280)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6.execute(StandardJanusGraphTx.java:1148)
at org.janusgraph.graphdb.query.QueryProcessor$LimitAdjustingIterator.getNewIterator(QueryProcessor.java:209)
at org.janusgraph.graphdb.query.LimitAdjustingIterator.hasNext(LimitAdjustingIterator.java:68)
at org.janusgraph.graphdb.query.ResultSetIterator.nextInternal(ResultSetIterator.java:54)
at org.janusgraph.graphdb.query.ResultSetIterator.(ResultSetIterator.java:44)
at org.janusgraph.graphdb.query.QueryProcessor.iterator(QueryProcessor.java:68)
at com.google.common.collect.Iterables$7.iterator(Iterables.java:613)
at org.janusgraph.graphdb.tinkerpop.optimize.JanusGraphStep.lambda$new$0(JanusGraphStep.java:69)
at org.apache.tinkerpop.gremlin.process.traversal.step.map.GraphStep.processNextStart(GraphStep.java:139)
at org.apache.tinkerpop.gremlin.process.traversal.step.util.AbstractStep.hasNext(AbstractStep.java:143)
at org.apache.tinkerpop.gremlin.process.traversal.step.util.ExpandableStepIterator.hasNext(ExpandableStepIterator.java:42)
at org.janusgraph.graphdb.tinkerpop.optimize.JanusGraphPropertiesStep.initialize(JanusGraphPropertiesStep.java:87)
at org.janusgraph.graphdb.tinkerpop.optimize.JanusGraphPropertiesStep.processNextStart(JanusGraphPropertiesStep.java:106)
at org.apache.tinkerpop.gremlin.process.traversal.step.util.AbstractStep.hasNext(AbstractStep.java:143)
at org.apache.tinkerpop.gremlin.process.traversal.util.DefaultTraversal.hasNext(DefaultTraversal.java:192)
at org.apache.tinkerpop.gremlin.console.Console$_closure3.doCall(Console.groovy:234)
...
Caused by: org.janusgraph.core.JanusGraphException: Could not call index
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6$6.call(StandardJanusGraphTx.java:1273)
at org.janusgraph.graphdb.query.QueryUtil.processIntersectingRetrievals(QueryUtil.java:255)
... 65 more
Caused by: org.janusgraph.core.JanusGraphException: Could not execute operation due to backend exception
at org.janusgraph.diskstorage.util.BackendOperation.execute(BackendOperation.java:57)
at org.janusgraph.diskstorage.BackendTransaction.executeRead(BackendTransaction.java:469)
at org.janusgraph.diskstorage.BackendTransaction.indexQuery(BackendTransaction.java:413)
at org.janusgraph.graphdb.database.IndexSerializer.query(IndexSerializer.java:547)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6$6$1.lambda$call$0(StandardJanusGraphTx.java:1269)
at org.janusgraph.graphdb.query.profile.QueryProfiler.profile(QueryProfiler.java:97)
at org.janusgraph.graphdb.query.profile.QueryProfiler.profile(QueryProfiler.java:89)
at org.janusgraph.graphdb.query.profile.QueryProfiler.profile(QueryProfiler.java:81)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6$6$1.call(StandardJanusGraphTx.java:1269)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6$6$1.call(StandardJanusGraphTx.java:1266)
at com.google.common.cache.LocalCache$LocalManualCache$1.load(LocalCache.java:4742)
at com.google.common.cache.LocalCache$LoadingValueReference.loadFuture(LocalCache.java:3527)
at com.google.common.cache.LocalCache$Segment.loadSync(LocalCache.java:2319)
at com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2282)
at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2197)
at com.google.common.cache.LocalCache.get(LocalCache.java:3937)
at com.google.common.cache.LocalCache$LocalManualCache.get(LocalCache.java:4739)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx$6$6.call(StandardJanusGraphTx.java:1266)
... 66 more
Caused by: org.janusgraph.diskstorage.PermanentBackendException: Could not open index reader on store: edges
at org.janusgraph.diskstorage.lucene.LuceneIndex$Transaction.getSearcher(LuceneIndex.java:773)
at org.janusgraph.diskstorage.lucene.LuceneIndex$Transaction.access$000(LuceneIndex.java:753)
at org.janusgraph.diskstorage.lucene.LuceneIndex.query(LuceneIndex.java:440)
at org.janusgraph.diskstorage.indexing.IndexTransaction.query(IndexTransaction.java:102)
at org.janusgraph.diskstorage.BackendTransaction$6.call(BackendTransaction.java:416)
at org.janusgraph.diskstorage.BackendTransaction$6.call(BackendTransaction.java:413)
at org.janusgraph.diskstorage.util.BackendOperation.executeDirect(BackendOperation.java:69)
at org.janusgraph.diskstorage.util.BackendOperation.execute(BackendOperation.java:55)
... 83 more
Caused by: org.apache.lucene.index.IndexFormatTooOldException: Format version is not supported (resource BufferedChecksumIndexInput(MMapIndexInput(path="/private/tmp/gods/luc/edges/segments_1"))): 3 (needs to be between 4 and 6). This version of Lucene only supports indexes created with release 5.0 and later.
at org.apache.lucene.codecs.CodecUtil.checkHeaderNoMagic(CodecUtil.java:213)
at org.apache.lucene.index.SegmentInfos.readCommit(SegmentInfos.java:304)
at org.apache.lucene.index.SegmentInfos.readCommit(SegmentInfos.java:288)
at org.apache.lucene.index.StandardDirectoryReader$1.doBody(StandardDirectoryReader.java:57)
at org.apache.lucene.index.StandardDirectoryReader$1.doBody(StandardDirectoryReader.java:54)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:692)
at org.apache.lucene.index.StandardDirectoryReader.open(StandardDirectoryReader.java:77)
at org.apache.lucene.index.DirectoryReader.open(DirectoryReader.java:63)
at org.janusgraph.diskstorage.lucene.LuceneIndex$Transaction.getSearcher(LuceneIndex.java:768)
... 90 more
```

There is some documentation suggesting that the [IndexUpgrader Tool](https://cwiki.apache.org/confluence/display/solr/IndexUpgrader+Tool) can upgrade an existing index to the current file format, but I ran into this after using the IndexUpgrader:

```
Caused by: java.lang.IllegalStateException: unexpected docvalues type NONE for field 'place__x' (expected=NUMERIC). Re-index with correct docvalues type.
at org.apache.lucene.index.DocValues.checkField(DocValues.java:212)
at org.apache.lucene.index.DocValues.getNumeric(DocValues.java:227)
at org.apache.lucene.spatial.vector.DistanceValueSource.getValues(DistanceValueSource.java:68)
at org.apache.lucene.queries.function.FunctionRangeQuery$FunctionRangeWeight.scorer(FunctionRangeQuery.java:163)
at org.apache.lucene.queries.function.FunctionRangeQuery$FunctionRangeWeight.scorer(FunctionRangeQuery.java:121)
at org.apache.lucene.search.Weight.scorerSupplier(Weight.java:126)
at org.apache.lucene.search.LRUQueryCache$CachingWrapperWeight.scorerSupplier(LRUQueryCache.java:726)
at org.apache.lucene.search.BooleanWeight.scorerSupplier(BooleanWeight.java:400)
at org.apache.lucene.search.BooleanWeight.scorer(BooleanWeight.java:381)
at org.apache.lucene.search.Weight.bulkScorer(Weight.java:160)
at org.apache.lucene.search.BooleanWeight.bulkScorer(BooleanWeight.java:375)
at org.apache.lucene.search.LRUQueryCache$CachingWrapperWeight.bulkScorer(LRUQueryCache.java:790)
at org.apache.lucene.search.ConstantScoreQuery$1.bulkScorer(ConstantScoreQuery.java:120)
at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:665)
at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:472)
at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:591)
at org.apache.lucene.search.IndexSearcher.searchAfter(IndexSearcher.java:576)
at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:503)
at org.janusgraph.diskstorage.lucene.LuceneIndex.query(LuceneIndex.java:448)
at org.janusgraph.diskstorage.indexing.IndexTransaction.query(IndexTransaction.java:102)
at org.janusgraph.diskstorage.BackendTransaction$6.call(BackendTransaction.java:416)
at org.janusgraph.diskstorage.BackendTransaction$6.call(BackendTransaction.java:413)
at org.janusgraph.diskstorage.util.BackendOperation.executeDirect(BackendOperation.java:69)
... 84 more
```

Deleting the `index.search.directory` and reindexing made it work again.

```
gremlin> mgmt = graph.openManagement()
==>org.janusgraph.graphdb.database.management.ManagementSystem@4dde8976
gremlin> idx = mgmt.getGraphIndex('edges')
==>edges
gremlin> mgmt.updateIndex(idx, SchemaAction.REINDEX)
==>org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor@76e3b45b
gremlin> mgmt.commit()
==>null
gremlin> org.janusgraph.graphdb.database.management.ManagementSystem.awaitGraphIndexStatus(graph, 'edges').status(SchemaStatus.ENABLED).call()
==>GraphIndexStatusReport[success=true, indexName='edges', targetStatus=[ENABLED], notConverged={}, converged={reason=ENABLED, place=ENABLED}, elapsed=PT0.001S]
gremlin> g.E().has('place', geoWithin( Geoshape.circle(37.6, 23.8, 20) )).values('place')
==>POINT (23.9 37.700001)
```

We'll need a migration document to capture this among other things.

Start with the reported Lucene IndexUpgrader behavior and the Gremlin management entry points `mgmt.updateIndex(idx, SchemaAction.REINDEX)` and `awaitGraphIndexStatus`. Document the format incompatibility, the failed upgrade symptom, and the successful delete-and-reindex procedure; done means the migration guidance covers this scenario clearly.

Written by the indexing model from the issue text.