mcreq_dump_packet: Provide string interpretations of flags

Change-Id: I9049ec7065ac4338899977d0014fab00b55c47e0
Reviewed-on: http://review.couchbase.org/42286
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>