Fix: liblttng-ctl comm: lttng_channel is not packed

Observed issue
==============

The size of the struct differs between bitness for x86-64 and x86
leading to serialization/deserialization problem across client
(liblttng-ctl) and lttng-sessiond.

sizeof(struct lttng_channel):

  x86: 608
  x86-64: 624

The struct cannot be marked as LTTNG_PACKED since it is part of the API.

Solution
========

Adopt a similar pattern to the new API with a "serialize" &
"create_from_buffer" approach. The only particularity is that we need to
flatten the channels on listing.

Most of the complexity is moved to `src/common/channel.c`

Known drawbacks
=========

None.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I59b0e92286e36c4f183d2950417cb180d4efc200