Request for initial review: Bucket types

Add a bucket type parameter to the following PB message:
- get
- put
- delete
- listkeys
- listbuckets
- 2i

Additionally, all references to 2index were changes to 2i. Perhaps this should have been done on a separate PR, but I needed up update 2i code anyways. Sorry for the noise this creates in the PR. 

I'd like to talk w/ @javajolt a bit more re: integration testing of each message, which I'll add in a later commit (hence the "request for initial review"). For now, existing integration tests take a `NULL` bucket type, indicating the `default` bucket.