Refactoring: introduce lttng_tracker_ids data structure

This data structure is opaque to allow for back-end implementation change
in the future.

For now, only the following functions concerning lttng_tracker_ids are public:
    lttng_list_tracker_ids
    lttng_tracker_ids_get_count
    lttng_tracker_ids_get_at_index
    lttng_tracker_ids_destroy

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: Iae1c10d0b578b402ab91378dd49f69f605b316b2
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>