plugins/common/muxing: compare stream class clock classes

plugins/common/muxing: compare stream class clock classes

When comparing two streams and all else is equal about the stream and
their classes, compare the clock classes attached to the stream classes.

My use case for this is that I'm writing a test where two source
components send a stream beginning message to a muxer components.  The
only difference between the two is the clock classes associated to the
stream classes.  Some stream classes have an associated clock class,
some don't.  In other cases, the stream classes differ only by some
properties of their clock classes.  I want to be sure that the messages
will come out of the muxer's priority heap in a stable order.

Change-Id: I113704dac4ff952573c3dd96acd33b295425a863
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12054
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>