ExecuteInTx PostgreSQL version (#1045)

* Enforce contract of ExecuteInTx at the API level

Previously ExecuteInTx accepted open transaction, but required
users never to execute any commands on it prior to calling
ExecuteInTx. This API change enforces this contract by making
ExecuteInTx to open transaction internally and pass it to the
callback func.

* Implement PG version of ExecuteInTx which does fewer roundtrips to the Server

PostgreSQL doesn't benefit from SAVEPOINT/ROLLBACK logic like CockroachDB
does. With this change Nakama checks server DB engine and enables CockroachDB
optimization only when necessary.

There are 2 behviour change in the PG version of ExecuteInTx:

- it retries on all "Class 40" (a.k.a retriable) codes, not just
  serialization error:

	40000 	transaction_rollback
	40002 	transaction_integrity_constraint_violation
	40001 	serialization_failure
	40003 	statement_completion_unknown
	40P01 	deadlock_detected

- It doesn't ignore COMMIT result code anymore