http://bugs.skysql.com/show_bug.cgi?id=453
Fixed bug in session command resul handling. In case where backend sent error message the session command cursor wasn't updated properly.

Added check to bref_clear_state, if bref's waiter counter would go negative, decrement to global operation counter is skipped.
Merge branch 'develop' of https://github.com/skysql/MaxScale into develop