Fix: source.ctf.lttng-live: muxing failure on clear (unit conversion)

Fix: source.ctf.lttng-live: muxing failure on clear (unit conversion)

This commit is a follow-up fix for 8ec4d5ff (see original message).
The original fix included a bogus comparison of:
  `stream_iter->last_inactivity_ts > curr_msg_ts_ns`

While the idea behind the fix is valid, this statement compares
nanoseconds since Unix Epoch (former) to clock cycles (latter).

A conversion of the `last_inactivity_ts` to nanoseconds since Unix epoch
is performed using the stream's default clock class allowing a
comparison in a common time base to take place.

The diff looks more intimidating than it really is as it shifts the
indentation of a lot of code at once. This is because we only want to
perform the timestamp conversion when necessary (very rarely) on this
fairly hot path.

Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibdc365fec4685da88ae141383d5e5ef0af169a87