Ability to efficiently use natural keys for event stream identification

[jeremydmiller]

By no means does this replace the surrogate keys, but also allow you to specify natural keys to event streams such that you can efficiently use FetchForWriting/FetchLatest/FetchStreamStateAsync w/ the natural key as well.

This comes up from time to time, but also would help for data coming externally. A JasperFx client could use this in their system to get around some oddball usage from their client.

Probably means opt in changes to the database storage.