coreapi: replace coreiface.DagAPI with ipld.DAGService

[magik6k]

As api.Dag() is the interface to IPLD, this feels like the right thing to do.

This wasn't done this way before because we keep format parsers in a global map in core/coredag, exposing ipld interfaces directly would make it tricky for outside code to do things like parse json to dag-cbor, but from my experience most Go code which touches IPLD doesn't really need those parsers.

(Since this is mostly a yes/no decision, I'd like this to move relatively fast as it's blocking a chunk of work in go-ipfs-http-api)

[schomatis]

I don't have enough knowledge of the CoreAPI but this feels more natural to me.

[Stebalien]

I agree that this is the right direction. I'm going to merge this now as the interface in go-ipfs is unstable but we need to make sure that ipfs-interface-core matches.

[magik6k]

make sure that ipfs-interface-core matches

Conceptually using go-ipld-format.DAGService is going to be closer to js/ipfs-interface-core, only the names are a bit different - put/Add, get/Get, tree/Resolver.Tree

[Stebalien]

SGTM.