condition: introduce lttng_capture_descriptor struct

The capture_index corresponds to the index at which the capture is
expected on reception of the trigger payload from the tracer.

This field will be populated on reception of the trigger on the sessiond
side and is internal use only.

This is necessary due to the fact that a single condition can have
multiple capture actions associated to it with capture descriptor
duplicate. We plan on forming an ordered set of these capture
descriptors and configure only the set for the enabler. This ensures
that we capture only what is required on the tracer side. On reception,
each capture descriptor will know the index at which the payload field
it corresponds to is situated.

In retrospect this is purely an optimization to prevent the tracer from
doing unnecessary work.

Initially this complexity was planned to be contained to the
lttng-sessiond, but since we now transfer the msgpack payload all the
way to liblttngctl we now have to carry this index all the way to
liblttng-ctl.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie3309511347818f88c61c750723f21c73d82f80b
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479