mds: fix race in connection accept; fix con replacement

mds: fix race in connection accept; fix con replacement

We solve two problems with this patch.  The first is that the messenger
will now reuse an existing session's Connection with a new connection,
which means that we don't want to change session->connection when we
are validating an authorizer.  Instead, set (but do not change) it.

We also want to avoid a race where:

 - mds recovers, replays Sessions with no con's
 - multiple connection attempts for the same session race in the msgr
 - both are authorized, but out of order
 - Session->connection gets set to the losing attempt's Connection*

Instead, we take advantage of an accept event that is called only for
accepted winners.

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