before_command() wait for ongoing rollbacks leaks

[sjaakola]

wsrep::client_state::before_command() has a wait loop to make the client execution to pause until external rollbacking of client's transaction has completed.

External rollbacker, sets clients transaction state to aborting, and the wait loop checks if transaction state is aborting:
while (transaction_.state() == wsrep::transaction::s_aborting)
{
cond_.wait(lock);
}
rollbacker sets the transaction state to aborted, in transaction::after_rollback(). This happens first, and later in rollbacker execution client_state::sync_rollback_complete() is called and the cond_ signal is sent then.
This sequence has a race condition: if rollbacker has called for transaction::after_rollback(), but not yet called client_state::sync_rollback_complete(), incoming client will pass the before_command() wait loop, and after that both client and rollbacker will operate on same client state until rollbacker completes.
This race condition accounts for some sporadic failures with multi-master conflict testing, e.g. with galera.galera_FK_duplicate_client_insert