mon: clean up ref counting for forwarded messages' sessions

Carry the refs across the duration of the session.  Explicitly pass
ownership to the MonSession.

Signed-off-by: Sage Weil <sage@inktank.com>
msg: factor policy handling out of SimpleMessenger

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: add convenience Connection methods

Signed-off-by: Sage Weil <sage@inktank.com>
osd: use Connection::send_message

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

At least in the obvious places.

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

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

Signed-off-by: Sage Weil <sage@inktank.com>
osdc/Objecter: use Connection::send_message()

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: remove Messsenger::send_message(..., Connection*)

Instead, make the Connection method pure virtual and implement it in the
PipeConnection specialization (which is specific to SimpleMessenger).

Signed-off-by: Sage Weil <sage@inktank.com>
mon: use send_message instead of lazy_send_message

We only use this for sending things to non-mons, and all of those Policy's
are server, so we will never initiate a connection.  This duplicates the
'lazy' semantics we were after.

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

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

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

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonClient: use Connection::mark_down()

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonClient: use Connection::send_keepalive()

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

Signed-off-by: Sage Weil <sage@inktank.com>
osdc/Objecter: use Connection::mark_down()

Signed-off-by: Sage Weil <sage@inktank.com>
msg: drop Messenger::mark_down() and send_keepalive() con-based calls

All users must stick to the Connection-based calls.

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