Closing JanusGraph intermittently gets into an endless loop if there is an FDB error

We have been using this adapter with JanusGraph for quite a while and have not seen issues, until we started encountering this. We have a job that usually succeeds and terminates, but occasionally runs forever with this error output over and over:

```
[pool-37-thread-1] WARN org.janusgraph.diskstorage.log.kcvs.KCVSLog - Could not read messages for timestamp [2021-05-11T02:21:21.872Z] (this read will be retried)
org.janusgraph.core.JanusGraphException: Could not execute operation due to backend exception
at org.janusgraph.diskstorage.util.BackendOperation.execute(BackendOperation.java:56)
at org.janusgraph.diskstorage.util.BackendOperation.execute(BackendOperation.java:158)
at org.janusgraph.diskstorage.log.kcvs.KCVSLog$MessagePuller.run(KCVSLog.java:725)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: org.janusgraph.diskstorage.PermanentBackendException: Permanent exception while executing backend operation messageReading@0:0
at org.janusgraph.diskstorage.util.BackendOperation.executeDirect(BackendOperation.java:81)
at org.janusgraph.diskstorage.util.BackendOperation.execute(BackendOperation.java:54)
... 9 more
Caused by: java.util.concurrent.CompletionException: com.apple.foundationdb.FDBException: Broken promise
at java.util.concurrent.CompletableFuture.reportJoin(CompletableFuture.java:375)
at java.util.concurrent.CompletableFuture.join(CompletableFuture.java:1947)
at com.apple.foundationdb.RangeQuery$AsyncRangeIterator.hasNext(RangeQuery.java:250)
at org.janusgraph.diskstorage.foundationdb.FoundationDBRecordIterator.fetchNext(FoundationDBRecordIterator.java:64)
at org.janusgraph.diskstorage.foundationdb.FoundationDBRecordAsyncIterator.fetchNext(FoundationDBRecordAsyncIterator.java:66)
at org.janusgraph.diskstorage.foundationdb.FoundationDBRecordIterator.hasNext(FoundationDBRecordIterator.java:47)
at org.janusgraph.diskstorage.util.StaticArrayEntryList.of(StaticArrayEntryList.java:390)
at org.janusgraph.diskstorage.util.StaticArrayEntryList.ofStaticBuffer(StaticArrayEntryList.java:345)
at org.janusgraph.diskstorage.keycolumnvalue.keyvalue.OrderedKeyValueStoreAdapter.convert(OrderedKeyValueStoreAdapter.java:145)
at org.janusgraph.diskstorage.keycolumnvalue.keyvalue.OrderedKeyValueStoreAdapter.getSlice(OrderedKeyValueStoreAdapter.java:69)
at org.janusgraph.diskstorage.log.kcvs.KCVSLog$MessagePuller$1.call(KCVSLog.java:791)
at org.janusgraph.diskstorage.log.kcvs.KCVSLog$MessagePuller$1.call(KCVSLog.java:788)
at org.janusgraph.diskstorage.util.BackendOperation.execute(BackendOperation.java:147)
at org.janusgraph.diskstorage.util.BackendOperation$1.call(BackendOperation.java:161)
at org.janusgraph.diskstorage.util.BackendOperation.executeDirect(BackendOperation.java:68)
... 10 more
Caused by: com.apple.foundationdb.FDBException: Broken promise
at com.apple.foundationdb.NativeFuture.Future_getError(Native Method)
at com.apple.foundationdb.FutureResults.getIfDone_internal(FutureResults.java:38)
at com.apple.foundationdb.FutureResults.getIfDone_internal(FutureResults.java:25)
at com.apple.foundationdb.NativeFuture.marshalWhenDone(NativeFuture.java:63)
... 3 more
```

This happens after our class performs a `System.exit`. In succeeded runs, the StandardJanusGraph closes all the instances of the adapter successfully, and in this case it closes all but one.

We have tried quite a few things with no success, so wanted to reach out here to see if anyone else has seen it or has any insights. With isolation level `read_committed_with_write`, the adapter retries only three times as expected, but then this error is thrown and it starts again.

Our current Janus storage settings are:
```
storage.backend=org.janusgraph.diskstorage.foundationdb.FoundationDBStoreManager
storage.lock.retries=5
storage.lock.wait-time=250
storage.fdb.cluster-file-path=/etc/foundationdb/fdb.cluster
storage.fdb.isolation-level=serializable
storage.fdb.get-range-mode=iterator
```
and we are using JanusGraph `0.5.3` and FDB `6.2.22`. We also tried isolation-level `read_committed_with_write` and range mode `list`.

Start by tracing the retry path from KCVSLog$MessagePuller through BackendOperation and the FoundationDBRecordIterator classes named in the stack trace. Compare the serializable and read_committed_with_write settings and the iterator and list range modes described in the issue. Done means the reported FDB error no longer leaves one adapter instance retrying indefinitely after shutdown.

Written by the indexing model from the issue text.