mysql: fix lost context related to pool.getConnection
mysql: fix lost context related to pool.getConnection

This fix deals with a very specific case and fixes the test case added
in commit 9878643:

1. Prior to starting a transaction, create a connection using
   pool.getConnection()
2. Release the connection using .release()
3. Start a transation using opbeat.startTransaction()
4. Get a new connection using pool.getConnection()
5. At this point the currentTransaction would be lost

opbeat/opbeat-node,opbeat/opbeat-node