test/ceph_test_msgr: do not use Message::middle for holding transient data

test/ceph_test_msgr: do not use Message::middle for holding transient data

Message::middle is used for holding encoded data, so we we can not stuff
it with payload and leave the "payload" field empty. this change
refactors the ceph_test_msgr by introducing a Payload class which
encodes all test data in it.

Fixes: http://tracker.ceph.com/issues/17728
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 56896a7ed20869ce91ade4c77c1d6cbab8d50de1)
Conflicts:
	src/test/msgr/test_msgr.cc: do not use the new-style DENC()
framework for implementing the encoder of Payload class. DENC() was
introduced after jewel was released.