Adapt to LTTng-UST ABI/control API updates

Adapt to LTTng-UST ABI/control API updates

Adapt to ABI/control API updates introduced by LTTng-UST commit
24f7193c9b91 ("Introduce extension points for trace hit counters")

Note that this commit reverts back to uint32_t for channel event ids.

UST notification LTTNG_UST_CTL_NOTIFY_CMD_KEY is currently handled as
-LTTNG_UST_ERR_NOSYS (TODO).

The type of the argument to is_max_event_id() is changed to
lttng::sessiond::ust::event_id to match the newly introduced event_id
type definition. Likewise for the registry_channel _next_event_id
member.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6299b07c02d35b067aed5e94f11353d3cc14f24c