sessiond: generate trigger name: name triggers with the 'trigger' prefix

sessiond: generate trigger name: name triggers with the 'trigger' prefix

Generated trigger names currently have the form TN, where N is the
number of generated trigger names over the lifetime of the session
daemon.

The form 'triggerN' seems more in line with autogenerated names such
as channel names (e.g. 'channel0').

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id61cd4716bb4c080d9242853c366e14542f60f7c