event-rule: lttng_event_rule base object

event-rule: lttng_event_rule base object

A lttng_event_rule object is the base object representing an event-rule.

We plan on using the event-rule object for compositing with a new
condition type.

This also paves the way toward further improvement of the lttng_event
related APIs.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5bbfa3b75337f040cf7e565447d4a4af590ed043