bt2: add trace collection notification iterator

A trace collection notification iterator is a special notification
iterator which iterates on the multiplexed notifications of all the
output ports of one or more traces. The new notification iterator
supports a stream intersection mode and trimming the whole multiplexed
stream.

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