sessiond: trigger: decouple reception of triggers from commands

sessiond: trigger: decouple reception of triggers from commands

The trigger to register/unregister is received in the body of the
register and unregister commands.

Isolate the reception of the trigger from the command payload in
a util in client.c (receive_lttng_trigger()) which allows the commands
to only operate on fully-formed objects.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iaee6c54590e259ce053d368494ff0c8b356a36a1