retry autocommit mode can retry also MST

[Seppo Jaakola]

If wsrep_retry_autocommit option is set, then autocommit mode transactions, which were aborted by BF applier, will be retried.
The logic to test for autocommit mode is:

      if (!(thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)) &&
          wsrep_retry_autocommit)

However, before we reach this state, the execution has passed end_trans(), where some
thd->options have been cleared:

    thd->options&= ~(OPTION_BEGIN | OPTION_KEEP_LOG);

As a consequence, if session was in autocommit mode, and started transaction with explicit BEGIN, then
autocommit retry logic will retry just the last statement of the aborted multi statement transaction.

[Seppo Jaakola (seppo-jaakola)]

A quick fix, with little interference with rest of the code, has been committed.
There is new THD variable for telling if autocommit retry can be tried for this session.

THD::wsrep_retry_autocommit is set before dispatch_command().

A larger refactoring of aborting state model could also be considered.