Support for playback of old events.

Introduces the time_sync object which can sync
time with an external time service (like the time_sync
service in notigen).

This is used for playback of old events. It prevents
premature expiry triggers.

Also some tweaks to the UsageHandler, work in progress
and used for testing.

Change-Id: I45033fbd9c12d98f96816a4f90cf7dc8c915ef51