Fix to #478, http://bugs.skysql.com/show_bug.cgi?id=478
Execution of session commands failed because session commands and normal sql statements were executed in a wrong order if backend was executing previous session command while new sql stmt was routed to that backend. There was a window where ordering went wrong.
It is possible that one normal sql stmt arrives while previous sescmds are still being executed.

Introduced a new member in backend_ref_t structure, bref_pending_cmd where new sql stmt pointer is stored in that case. When sescmds are executed completely, that command is automatically executed next.
Merge branch 'blr' of https://github.com/skysql/MaxScale into blr