ctf: make src.ctf.fs handle a single trace

ctf: make src.ctf.fs handle a single trace

Now that traces with different UUIDs get grouped in different src.ctf.fs
instances, it's not (as) useful for one src.ctf.fs component to support
reading multiple traces at the same time.

This patch simplifies the src.ctf.fs component class to make it handle
only a single logical trace (which can actually be composed of multiple
traces with the same UUID).

Internally, it works the same way as before: we create one `struct
ctf_fs_trace` per input path.  We then merge all of them to a single
one, the one that has the most expansive metadata.

Change-Id: I5a395cac3c379dedc06d98ccffa02b3750a35173
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1993
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>