It has a few implications to launch a raft peer when you wanted to do crdt and vice-versa.
Same when exporting and exporting states.
Users starting cluster peers should be explicit about their consensus choice.
Also, if we ever want to make `crdt` the default, we can't do that before
making `raft` non-default first. I don't like to break things but otherwise
the experience for new users wanting to try crdts might be aweful.
