sessiond: notification: add support for async commands

sessiond: notification: add support for async commands

Notification thread commands are currently all blocking. This means
that the emitter of the command uses the notification_thread_command
APIs to (stack) allocate a command context and wait on an lttng_waiter
for a reply.

In preparation for the addition of non-blocking commands, a new type
of asynchroneous commands is introduced. Asynchroneous commands are
"fire-and-forget"; the caller has no way of obtaining the result of
the command.

Asynchroneous command contexts are heap-allocated and their ownership
is transferred to the notification thread. The notification thread
will not attempt to wake-up the emitter and will free() the command
context regardless of the command's result.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6951987dda0262d1500a3b4e1403fb2559a3ff44