CCBC-243 Handle SASL STEP and LIST_MECHS commands

CRAM-MD5 mechanism requires handlers for PROTOCOL_BINARY_CMD_SASL_STEP
and PROTOCOL_BINARY_CMD_SASL_LIST_MECHS

Change-Id: I117d751cff2b5ddf09f9a08d82b868a7b91e785a
Reviewed-on: http://review.couchbase.org/29628
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>