Decouple stream id type from event id type

[tomasaschan]

It might make sense to allow e.g. string for stream id, while using e.g. Guid or long for the event id.

Example use case:

• You have an existing database that uses long for ids, so you want to use long for the event id as well.
• You want to refer to your streams with some composite information, e.g. aggregate type + id. The easiest way to achieve that would be to use e.g. "MyAggregate-123" as the event stream for an aggregate of type MyAggregate with id 123.