Storage interface should be kvstore, not "storeSignedPreKey" etc

[tristanhoy]

I can't see any reason why this signal-specific logic shouldn't be a part of the library itself.

The storage interface should simply have put/get/remove, allowing MUCH simpler integration with various storage backends.

[stwiname]

The problem with a KV store is that they often only let you store primitive data type. e.g. LocalStorage only lets you store strings.

Being able to know the data you are storing makes it much easier to convert between string and the desired object