relayd: create sessiond trace chunk registry on session creation

Use the sessiond_trace_chunk_registry_session_created() and
sessiond_trace_chunk_registry_session_destroyed() to allow the
sessiond trace chunk registry the ability to manage trace chunk
registries associated with the various session daemons from which
the consumer daemon connections originate.

These notifiers effectively manipulate the reference count (and
create/destroy, as needed) of the lttng_trace_chunk_registry
associated with each session daemon (using their UUID).

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