Fixes symtoms described in http://bugs.skysql.com/show_bug.cgi?id=454
Added DCB state check to backend error evend handler. If state isn't DCB_STATE_POLLING anymore, socket is closed with purpose or error occurred but it has already been handled.
Client doesn't hang anymore at least with the documented method.
Merge branch 'develop' of https://github.com/skysql/MaxScale into develop