relayd: viewer-stream: introduce viewer_stream_close_files and viewer_stream_sync_tra...

relayd: viewer-stream: introduce viewer_stream_close_files and viewer_stream_sync_tracefile_array_tail

Introduce the following viewer helpers for clear:

- viewer_stream_close_files: Allow closing currently open viewer files.

- viewer_stream_sync_tracefile_array_tail: Allow updating the current
  tracefile id and the index sent seqcount based on the current content
  of the tracefile array.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I5d2f72868625b00b207c75c202bc514768c35166
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>