Consider expanding the core Blockstore interface

Lotus, and by extension a few other blockstores currently use [the following interface](https://github.com/filecoin-project/lotus/blob/6afa99d0a1/blockstore/blockstore.go#L14-L35).
```
type Blockstore interface {
blockstore.Blockstore
blockstore.Viewer
BatchDeleter
Flusher
}
```

With the current plan of cloning go-libipfs with unforced migration-by-deprecation the time is right to augment the core interface to be just like the above.

For completeness here is [how lotus deals with the transition](https://github.com/filecoin-project/lotus/blob/6afa99d0a1/blockstore/blockstore.go#L108-L150)