sessiond: notification: refactor: split transmission and poll update

Split the notification transmission logic and its effect on a
notification_client from the logic tied to the management of the
notification thread.

This is to make it possible to send (or queue) notifications from the
notification thread or another thread. If another thread encounters an
error or a full socket buffer, a future mechanism will allow it to
signal the notification thread to update its private state (e.g. poll
mask).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I35d8943cb11473d82b07a4dbc5a0f093cde25a79