CCBC-435: retrieve command _after_ "purge_implicit_responses"

Change-Id: I126f7ef350cb983567ef923d33c83c43b8ca24d8
Reviewed-on: http://review.couchbase.org/37712
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>