Change place/transition identifier types from Long to String

[merlijn]

Places and Transitions in the petri net are currently identified by a Long number. This identifier is used in a number of commands and messages send to/from the ProcessInstance actor.

In hindsight this choice proved to be mistake because it is not easily used in the DSL or understandable by humans.

Also there is a potential hash collision problem since the id is not explicitly specified but computed from other properties.

In a Recipe interactions are given a "label" which is converted to a number using a hashcode function. Even tough a 64 bit SHA256 hashcode is used there can still be collisions.

The proposition is to re-use the "label" given to an interaction as it's transition identifier. The same goes for places.

[Tim-Linschoten]

We can not change this easily since we store it on event sourcing and we want to be backwards compatible.
Next to this the chance of collisions is almost zero since not many places and transitions is limited.