Support for VertexPrograms

Background: we used earlier janusgraph+hbase and now we want to move towards foundation db on k8. So we are trying out the janusgraph-foundation db adaptor

During the tests, we figured out that our existing algorithms, similar to the link below
https://github.com/apache/tinkerpop/blob/master/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/ranking/pagerank/PageRankVertexProgram.java

is failing due to the below errors

```
08:47:48 ERROR org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor - Exception trying to setup the job:
org.janusgraph.diskstorage.PermanentBackendException: Permanent failure in storage backend
at com.experoinc.janusgraph.diskstorage.foundationdb.FoundationDBKeyValueStore.getSlice(FoundationDBKeyValueStore.java:137) ~[janusgraph-foundationdb-0.1.0.jar:na]
at org.janusgraph.diskstorage.keycolumnvalue.keyvalue.OrderedKeyValueStoreAdapter.getKeys(OrderedKeyValueStoreAdapter.java:116) ~[janusgraph-core-0.4.1.jar:na]
at org.janusgraph.diskstorage.keycolumnvalue.KCVSUtil.getKeys(KCVSUtil.java:70) ~[janusgraph-core-0.4.1.jar:na]
at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor.addDataPuller(StandardScannerExecutor.java:92) ~[janusgraph-core-0.4.1.jar:na]
at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor.run(StandardScannerExecutor.java:121) ~[janusgraph-core-0.4.1.jar:na]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_201]
Caused by: org.janusgraph.diskstorage.PermanentBackendException: Max transaction reset count exceeded
at com.experoinc.janusgraph.diskstorage.foundationdb.FoundationDBTx.getRange(FoundationDBTx.java:188) ~[janusgraph-foundationdb-0.1.0.jar:na]
at com.experoinc.janusgraph.diskstorage.foundationdb.FoundationDBKeyValueStore.getSlice(FoundationDBKeyValueStore.java:129) ~[janusgraph-foundationdb-0.1.0.jar:na]
... 5 common frames omitted
Exception in thread "Thread-4" java.lang.NullPointerException
at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor.cleanup(StandardScannerExecutor.java:224)
at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor.cleanupSilent(StandardScannerExecutor.java:235)
at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor.run(StandardScannerExecutor.java:125)
at java.lang.Thread.run(Thread.java:748)
```

Has anyone faced similar errors before? appreciate your help. thanks.

Start by comparing the TinkerPop PageRankVertexProgram.java implementation with the FoundationDB adapter path shown in FoundationDBKeyValueStore.getSlice and FoundationDBTx.getRange. Reproduce the scanner failure and trace the Max transaction reset count exceeded error through StandardScannerExecutor. Done is not defined by the issue; investigation would first need to identify the adapter incompatibility and agree on a fix.

Written by the indexing model from the issue text.