Rework generation of the VID

[zorancv]

This PR changes the generation of the VID in such a way that entities have an order determined by the order of the blockchain transactions from which they originate, rather than to be ordered by the current implementation of the graph-node and the VID autogenerated by the database. This in turn will ensure that the dependant subgraphs would have a deterministic order of their input entities and it would be consistent across different graph-node deployments producing the same POI.

[zorancv]

A general question: we already require that every Entity has an id, if it doesn't, things will fail in funky ways. Wouldn't it be simpler to just treat the vid as another attribute of Entity, i.e., require that entity.get("vid") always returns something instead of keeping it separate from the entity?

@lutter I've decided to go that route. It simplifies the code a lot. Also I've added a check in the EntityCache set() that there was no VID previously set.

Finally when deserializing the entities from the database, I added deserialization of the the VID in addition to the list of fields from InputSchema, in order to keep the presence of the VID consistent in the Entity struct.

Those are two major changes since your last review.