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

https://issues.jboss.org/browse/ISPN-4651
Init is called only once, start/stop can be called multiple times - and I don't see why we should reallocate the semaphore. 
You would have to release the threads that could be waiting for the old semaphore in start anyway, therefore, I think that this is the most simple & working way to code that.