Fix to bug #590, http://bugs.mariadb.com/show_bug.cgi?id=590
Added function poll.c:poll_add_epollin_event_to_dcb which adds given event and related GWBUF to given DCB. The DCB is then added to event queue list so that its even becomes executed as soon as any of the threads start to process events next time.
The problem solved is case where COM_CHANGE_USER failed due to authentication failure. An error should be sent to the client but it was sent once per failed backend authentication instead of just once.
Thread which failed in authentication bypassed router and thus replies weren't filtered out. Solution adds an event and a GWBUF including error message to DCB and the DCB is then added to event queue. Thus the error message appeares like it was an incoming event and becomes processed properly.

An addition was made to mysql_backend.c:gw_read_backend_event so that self-generated event can be processed. Even if socket read doesn't manage to read anything, function proceeds if there is enough data in DCB's readqueue.

Fix ti bug #602, http://bugs.mariadb.com/show_bug.cgi?id=602
Added function sescmd_response_complete which checks if the read data completes session command response packet(s). Function is called only if there's a sessoin command being executed.
Merge branch 'develop' of https://github.com/mariadb-corporation/MaxScale into develop