Refactoring: remove ring buffer channel pointer from struct lttng_ust_channel_buffer

This pointer can be moved to an internal data structure, and be
populated by the ring buffer client rather than the probes.

Also transition all ring buffer client callbacks to use buffer and
channel structures rather than shm handle.

Also remove the handle from struct ustctl_consumer_stream (internal to
ustctl.c) because it is redundant.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I9c389626de5a02cf2ac7377d9f1e2802128992c4