sessiond: introduce ltt_session::locked_ref look-up functions

sessiond: introduce ltt_session::locked_ref look-up functions

Introduce ltt_session::locked_ref look-up functions and use them at the various
sites performing session look-ups by id or name.

The users of those look-up functions are refactored to make them exception-safe.
The biggest change is that process_client_msg is now exception-safe. The
functions are also moved inside of the ltt_session class as static methods. This
namespaces them implicitly.

A number of functions that expect a locked ltt_session are modified to accept an
ltt_session::locked_ref, thus enforcing the locking assumptions of the session
at compile time.

Change-Id: If9e4f8b25d03fa8d36a5898dd421da947ec4030c
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>