relayd: close viewer stream trace chunk earlier on release

A viewer stream puts its references to its stream and index files
within its "release" method (called when its reference count reaches
0).

However, the reference to its trace chunk is only released during the
RCU reclamation of the viewer stream. This unnecessarily delays the
clean-up of the viewer trace chunk.

For cleanliness' sake, move the release of the viewer stream's trace
chunk to the release method, just after the release of the various
file handles of that stream.

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