bitswap: Use multihashes, not CIDs, when checking if a CID is in a wantlist.

#### Version information:

> go-ipfs version: 0.4.11-dev-48476b292
Repo version: 5
System version: amd64/linux
Golang version: go1.8.3

#### Type:

Enhancement

#### Severity:

Implementation: Low
Figuring this out: High

#### Description:

Currently, we bitswap using CIDs. This means that, for example, if a peer asks for a CIDv1 thing that we store as a CIDv0 thing, we won't be able to give it to them. This is going to be a problem if we decide to switch to base32 as the default multihash encoding.

~See https://github.com/ipfs/notes/issues/259 for more design related discussion (this issue is to track the actual bug in go-ipfs).~