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 
Ah, I see that we want to keep args in cmd. But you still need to add it to raftCmd.

Also, let's rename storage.Cmd to storage.pendingCmd. No reason this should be exported, and I think the name should change to be a bit more descriptive and differentiated by purpose from raftCmd.Cmd