feat(daemon) `ipfs daemon --initialized`

With this PR, users can fetch and run ipfs in _one_ command.

```
docker run -d -p 4001:4001 -p 5001:5001 jbenet/go-ipfs daemon --initialized
```

Magic.

nb: needs https://github.com/jbenet/go-datastore/pull/9
(To sum up, yes the interface needs work, it should be clearer, and less dangerous to override. but _what it's actually doing_ is not likely to change.) Btw, i'm drafting the new version here: https://godoc.org/github.com/jbenet/go-ctxgroup