Handle edge cases and support autocommit=false

[luneo7]

Handles #14

Only tries to disable autocommit if needed.

Also handles edge cases, we internally hit a bug in where in the catch block we were calling conn.setAutoCommit(true); and there it failed, so the finally block condition if (null != didWeStartWork.get() && conn.getAutoCommit()) { wasn't true, and that made the connection not get released back to the connection pool, since the connection is released only during the ending of the unit of work.

So I've simplified the logic to have everything in a try catch finally block and now we are always releasing the connection to the pool.

Improved the readTransactionMetadata cache, as there it is a concurrent hash map, so get doesn't block, so if it is a cache miss, concurrent calculations will use the compute if absent, which synchronizes the calls, so we don't perform multiple calculations for the same thing at once.

Reduced the number of thread locals, and fixed it as ideally it should be a static field.

Also fixed a possible leak as if closing a the connection were to throw an error the thread local wouldn't never be removed.

[luneo7]

/cc @supercargo