fs-sink: trace static handling + cleanup teardown

When a trace is static and we have seen the stream_end notification for
all the active streams, we can free the resources for handling the
trace.

This also highlighted that a few bits were missing for a clean teardown
of the plugin.

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