trigger: implement trigger naming

A trigger can now have an optional name on the client side.

If no name is provided the sessiond will generate a name and return a
trigger object to populate the client side object.

For now, the name generation code generate the following pattern: TN

Where `N` is incremented each time a name has to be generated. If a
collision occurs, we increment `N` as needed.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5f303610713c049177e53937bfc9824cd61501e4