[RFC] Dis-entangle number generator from sequence.

This is a request for comment on modifying the sequence bundle/component. It seems that we are currently mixing two concepts: (order-)number generation vs. sequence generation. 

Currently the GeneratorInterface requires a Sequence entity to be passed, but looking at the HashGenerator it is obvious that there are generator strategies which don't require a sequence (hash, uuid, timestamp-based, pre-generated, etc), so I don't think the GeneratorInterface should mandate the use of a Sequence.

In this proposed change, only the SequentialGenerator relies on the Sequence. This also frees up the NumberListener from creating the Sequence, even though it may not be used..

~~Please note that I have not adjusted the tests yet, this is just a request for comments.~~ Tests adjusted.

Thank you.
Separating the hash-generator from the Sequence concept was one of my main goals here. I feel this is where this bundle mixes concerns too much already (see previous comment about number generator vs. sequence provider).
One could want UUID generator or a generator based on some ERP api, and those wouldn't need a sequence either, so I think it's wrong to tie these concepts together.