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.
* Daemon: support remote configuration
This:
* Adds support for fetching the configuration from a remote HTTP location:
`ipfs-cluster-service init http://localhost:8080/ipfs/Qm...` will instruct
cluster to read the configuration file from ipfs on start (potentially making
use of ipns and dnslink).
This is done by creating a `service.json` like `{ "source": <url> }`.
The source is then read when loading that configuration every time the daemon starts.
This allows to let users always use a mutating remote configuration, potentially
adding/removing trusted peers from the list or adjusting other things.
* Configuration and state helpers from ipfs-cluster-service have been extracted
to its own cmdutils package. This will help supporting something like an
`ipfs-cluster-follow` command in the next releases.
* Allows to disable the rest api by not defining it in the configuration (I thought
this was already so, but apparently only affected the ipfsproxy).
* Removes informer/allocator configurations from the daemon (--alloc). No one used
a non default pair. In fact, it was potentially buggy to use the reposize one.
