feat(*)!: Adds cas operation and fixes keys cursor

[thomastaylor312]

This PR incorporates some feedback received on the current draft interface. First off, this changes the cursor for listing keys to be a string instead of a number in order to support more systems.

The second changes adds back in a CAS operation as multiple people had been asking for it. See the discussion in #44 for more context.

Because these are breaking changes and there are multiple draft implementations out there, I bumped this to draft2. Ideally we should be able to move to an RC soon with these changes added in

Closes #44 Closes #45