msgr: take over existing Connection on Pipe replacement

msgr: take over existing Connection on Pipe replacement

If a new pipe/socket is taking over an existing session, it should also
take over the Connection* associated with the existing session.  Because
we cannot clear existing->connection_state, we just take another reference.

Clean up the comments a bit while we're here.

This affects MDS<->client sessions when reconnecting after a socket fault.
It probably also affects intra-cluster (osd/osd, mds/mds, mon/mon)
sessions as well, but I did not confirm that.

Backport: argonaut
Signed-off-by: Sage Weil <sage@inktank.com>