Clean-up: sessiond: move registry_session free functions under class

Clean-up: sessiond: move registry_session free functions under class

Move a number of registry_session methods implemented as c-style
free functions under the `registry_session` class. This makes it
possible to make a large number of attributes private.

This leaves only the metadata storage and locking facilities
publicly accessible. Making them private requires a bit more
refactoring.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6bdd4cc8918e5d7025fc3d735fadbf1ce2e9b7c8