bt2: let Python message iterators implement seek beginning

bt2: let Python message iterators implement seek beginning

This patch adds the possibility for message iterators implemented in
Python to implement the "can seek beginning" and "seek beginning"
operations.

A message iterator can support "seek beginning" by defining a
_seek_beginning method:

  class MyIter(bt2._UserMessageIterator):
    def _seek_beginning(self):
      # Do the needful.

It can support the "can seek beginning" operation by defining a
_can_seek_beginning property, which must evaluate to a bool:

  class MyIter(bt2._UserMessageIterator):
    @property
    def _can_seek_beginning(self):
      # Do the needful, including returning a bool.

The behavior of the "can seek beginning" operation is made to mimic
the C API:

- If the iterator has a _can_seek_beginning attribute, it is used to
  determine whether the iterator can seek beginning.
- Otherwise, the presence or absence of a _seek_beginning method
  indicates whether it can.

Change-Id: I68c48b5cd30090bff833529cda9fa918c7e72b0b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1555
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>