Refactor storage package's raft interface.

It's still using a dummy implementation, but it no longer relies on
sending channels and mutable reply buffers through raft.

@spencerkimball @cockroachdb/developers 
We've been moving towards more type-safe methods (e.g. gogoproto unions), but for the sets like PublicMethods, ReadMethods, WriteMethods there's not really a better option than string keys.