flt.utils.muxer: handle all message types specifically

The get_msg_ts_ns() function in `muxer.c` was only handling event and
inactivity messages, making all other messages high priority by default.
However, other messages can have default clock snapshots too, so they
can be sorted.

For discarded events and packets, we use the beginning clock snapshot
as, for the same stream, it must be greater than or equal to the
previous message's clock snapshot (if any), and less than or equal to
the next message's clock snapshot, if any.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>