Fixed issue with unique relationship creation response codes in REST.
Removed the ability to lock relationships via NodeManager, part of the work to ensure that we never acquire locks before we've enlisted data sources.
Added additional HA logging.
Make OtherThreadExecutor implement Closeable to make orderly thread
shutdown easier.

o Callers can now use try-with-resources.
Make NestedTransactionLocksIT more readable.
Ensure that all tests close transactions reliably.

o Recent changes to database shutdown mean that we now wait for transactions
   to finish when shutting down. This mean that each of the many tests which did
   not close their transactions was hanging for 20 seconds before failing.