Fixes#865.
This makes the necessary changes so that consensu is selected on "init" with a
flag set, by default, to "crdt". This generates only a "crdt" or a "raft"
section, not both.
If the configuration file has a "raft" section, "raft" will be used to start
the daemon. If it has a "crdt" section, "crdt" will be used. If it has none or
both sections, an error will happen.
This also affects "state *" commands, which will now autoselect how to work
from the existing configuration.
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.
