bt2: Adapt test_trace.py and make it pass

bt2: Adapt test_trace.py and make it pass

test_trace.py is heavily modified, because most of the responsibilities
of Trace have been moved to TraceClass.  A number of concepts, such as
packet headers, also disappeared.

Similarly, trace.py loses a lot of weight.  Note that copy and equality
operations are removed, since we ruled that they were complex but not
particularly useful.

A trace implements abc.Mapping, and maps stream ids to stream objects.

A trace is created by "calling" a trace class, hence the addition of the
__call__ method to TraceClass.

The test_trace test creates some streams and requires to control the ids
of those streams, so the create_stream_class method and StreamClass type
are enhanced to support that.

Change-Id: If23089abfbcbe5f0064069f93bd91126f1549b62
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1290
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>