trigger: lttng_triggers: implement a container for multiple triggers

This container is exposed for the listing of triggers.

We also plan on using it internally in the sessiond for inter-thread
communication.

The current implementation is backed by a lttng_dynamic_pointer_array.

Caller of lttng_triggers_add is responsible for managing ownership via
ref-counting of the lttng_trigger object.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib541027a6d7d856daa746de5aa49f0002bbe036f