mon: set session_timeout when adding to session_map

mon: set session_timeout when adding to session_map

With msgr2, the session is added in Monitor::ms_handle_accept()
which is queued by ProtocolV2 at the end of handling CLIENT_IDENT
frame, before responding with SERVER_IDENT frame.  session_timeout
is 0 and gets set only in Monitor::ms_dispatch(), so if the session
trimming code in Monitor::tick() gets to the session before the peer
receives our SERVER_IDENT, handles it, sends the first message and
we receive it, the session is wrongly closed.

This doesn't happen with msgr1, because there the session is added in
Monitor::ms_dispatch(), upon receive of the first message (MSG_AUTH).

Fixes: https://tracker.ceph.com/issues/47697
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>