relayd: add the sessiond_trace_chunk_registry interface

relayd: add the sessiond_trace_chunk_registry interface

The sessiond_trace_chunk_registry interface allows the relay
daemon to maintain an association between multiple consumer daemon
connections originating from the same session daemon and a common
trace chunk registry.

This trace chunk registry allows separate, but related, connections
to manipulate the same trace chunk.

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