CCBC-474: Fix memory leaks for extended commands in sched_fail

This adds a new field in the mc_REQDATAEX structure containing an
optional pointer to a destructor function which is called on each packet
that has been failed out. This may be used to decrement a reference
counter and/or free the extended cookie.

Change-Id: If6470d7370a31eb06e3e6ebd0c8f982f17b32124
Reviewed-on: http://review.couchbase.org/40128
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>