Refactoring: Channel structures

Now that struct lttng_channel is separate from the consumer daemon
channel config message ABI layout, we can perform a significant amount
of refactoring:

- Namespace theses structures with lttng_ust_ prefix,
- Use struct_size extensibility scheme.
- Split fields into public/private structures,
- Split public and private structures into a child (ring buffer)
  and parent (common). This will allow introducing other channel
  children types in the future (e.g. counters).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I6acec4e4ffff721e6ea5698e64a063045e1c1022