FoundationDBTx restart() race condition

`getMultiRange` (which is synchronized) is being called which is locking the transaction. Then, a transaction reset is being triggered, likely by the time limit being exceeded by the FDB driver, on that same transaction which triggers a call to `restart()`, which is also synchronized. It can't get the lock so it's stuck and then that blocks everything.

Steps to reproduce:

`mvn -Dtest=FoundationDBOLAPTest clean package test`

This thread dump demonstrates the issue on lines 23 and 502.

[jgfdb-stack-dump.txt](https://github.com/JanusGraph/janusgraph-foundationdb/files/4982461/jgfdb-stack-dump.txt)

Start with FoundationDBTx#getMultiRange and restart(), then run `mvn -Dtest=FoundationDBOLAPTest clean package test` and inspect jgfdb-stack-dump.txt at lines 23 and 502. Trace the synchronized transaction/reset interaction; done means the reproduced blocking condition is resolved and the test passes.

Written by the indexing model from the issue text.