msgr: kill addr-based send_keepalive()

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: kill mark_down_on_empty()

No users!

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: add Connection::mark_disposable()

Signed-off-by: Sage Weil <sage@inktank.com>
mds: use Connection::mark_disposable()

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: remove Messenger::mark_disposable()

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