Fix: sessiond: cmd_add_ctx: leak of internal channel members

Fix: sessiond: cmd_add_ctx: leak of internal channel members

lttng_channel instances must be released using channel_attr_destroy.
However, an error path of cmd_add_ctx uses free() directly, which causes
internal structures of lttng_channel to be leaked.

Wrap the lttng_channel instance to use a unique_ptr which invokes
channel_attr_destroy on release.

Change-Id: I77443c8a57475437dbb11792869e70840680492f
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>