utils.muxer: expect specific clock class properties to mux

This patch makes a filter.utils.muxer component's notification iterator
set what properties it expects from future clock classes from the first
clock classes it checks. The possible expectations are:

* Expect absolute clock classes, any UUID.
* Expect non-absolute clock classes, without a UUID.
* Expect non-absolute clock classes, with a specific UUID.

With this patch you can make the component mux the streams of a single
trace of which the clock is not absolute, or has no UUID.

The `assume-absolute-clock-classes` parameter overrides all those
checks.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>