ISPN-4651 LevelDB crashes JVM when stop() is called concurrently with write()

https://issues.jboss.org/browse/ISPN-4651
Although this could lead to a different type of issue as well if a lot of requests get backed up.  Unfortunately deleting the LevelDB on a clear makes this more difficult because we could just have stop set an AtomicBoolean to prevent additional requests from coming (throw an exception) to prevent backup.