Add Namespace capability

[christhekeele]

This is a common and useful utility that frequently is poorly implemented or supported.

[christhekeele]

This is really just a subclass of #15, with an emphasis on keys. This suggests the serialization story needs to be a customizable pipeline, rather than single transformation.

[christhekeele]

Ideally you'd have one store you could access in different namespaces without needing to start another GenServer. This means the namespace settings would need to live in the calling Conn so this is blocked by #9.

[christhekeele]

Or this could live on the server and the store initialization logic could be mandated to be idempotent.

[christhekeele]

Probably the best option is to make a meta-store that applies the transformations and forwards it on to the desired one. You could point multiple meta-stores to the same underlying one for multiple namespaces in the same store... So this is more a subclass of #11.