bt2: make `TraceCollectionMessageIterator` not use an output port msg iter

bt2: make `TraceCollectionMessageIterator` not use an output port msg iter

Because there's a plan to drop the output port message iterator concept
altogether, make `TraceCollectionMessageIterator` not use any output
port message iterator.

Instead, a `TraceCollectionMessageIterator` instance adds a proxy sink
component (`_TraceCollectionMessageIteratorProxySink`) to its graph,
connecting it to the last filter component in the chain, and sharing
with it a list having a single item.

When the sink consumes a message from its upstream message iterator, it
places it in the shared list as the first item.
TraceCollectionMessageIterator.__next__() is changed so that it calls
Graph.run_once() to make the proxy sink consume, and then reads the
consumed message from the shared list.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4b561837a9c23d3d758ea089193cfdabf99fc27e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1820
Tested-by: jenkins <jenkins@lttng.org>