relayd: create stream files relative to a session's trace chunk

relayd: create stream files relative to a session's trace chunk

Like to the consumer daemon, the relay daemon now creates its
stream files through the lttng_trace_chunk interface. This
requires changes across the session daemon and relay daemon.

The changes to the relay daemon mainly target the file creation.
In the session daemon, the snapshot command has been heavily
modified to fully initialize the ltt_session's ust and kernel
consumer_outputs _before_ the snapshot trace chunk is created.

This way, the session is setup in the same way a regular network
streaming session would be, a temporary trace chunk is created to
contain the snapshot's content, the actual snapshot is taken, and
the session's original consumer_outputs are restored.

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