Add DB dialect autodetection and dialects MySQL/Oracle

remove TX_SERIALIZABLE as it is dangerous for the pooled connection and also not necessary
This isolation level is required for the timers to work correctly. The DB is used to make sure the timer is only executed on one node, the first node that updates the timer row 'wins' and gets to actually run the timer. If a lower isolation level is in use then it is possible that two different nodes with think they have updated the row. 

Thinking about it we may only need REPEATABLE_READ, but READ_COMMITED is definitely not ok.