Support timer boundary events

[npepinpe]

Enables usage of timer boundary events during workflow execution.

As this is the first issue to add boundary events to our execution model, it should implement the following:

1. Process boundary model definitions to an executable model
2. Activities should setup their boundary events triggers during the ELEMENT_READY phase: in this case, create all timers attached.
3. Activities should clear any boundary event trigger during the ELEMENT_TERMINATING or ELEMENT_COMPLETING phase: in this case, delete all timers attached.
4. A boundary event's ELEMENT_ACTIVATED phase should cancel the attachedTo activity if so configured
5. A boundary event should not process its ELEMENT_ACTIVATED lifecycle step if the activity it is attached to is completing/terminating (i.e.: step guard)
6. An element should only spawn a new token when completed, not when terminated.
7. When the timer configured by the boundary event is triggered, it should start the correct boundary event's lifecycle (e.g. write ELEMENT_READY for it).