msgr: add KEEPALIVE2 feature

msgr: add KEEPALIVE2 feature

This is similar to KEEPALIVE, except a timestamp is also exchanged.  It is
sent with the KEEPALIVE, and then returned with the ACK.  The last
received stamp is stored in the Connection so that it can be queried for
liveness.  Since all of the users of keepalive are already regularly
triggering a keepalive, they can check the liveness at the same time.

See #7888.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit d747d79fd5ea8662a809c5636dfd2eaaa9bf8f5d)

Conflicts:

	src/include/ceph_features.h