Refactor Messenger class family.

In preparation for XioMessenger and other new transports,
reorganize Messenger class family to remove SimpleMessenger
details the interface.

This change moves Connection to its own header, and moves Pipe
details to a new PipeConnection descdendant class, among other
changes.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
msgr: move PipeConnection out of Connection.h; make anon

Remove last reference to PipeConnection outside of SimpleeMessenger by
creating a create_anon_connection() method for Messenger.

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