* Add db version attribute to executions table for better CAS guarantee
* 1.9.x will by default use existing next event ID for CAS, but will also support the new DB version CAS
* 1.10.x will by default use DB version CAS
Last event ID is not enough for CAS operation protection: e.g. activity heartbeat / buffer events.
The newly introduced DB version will be increased by 1 each time execution table is updated (update to mutable state record).
