Align tipset identifier with the FIP

[anorth]

The FIP at present specifies tipset identifiers as []Cid in message. The code instead uses an opaque string (byte array). CIDs are opqaue to the protocol so there's not a good reason to depend on a CID package or import any semantics. But since CID has a specific IPLD tag, there's a mismatch to be bridged somewhere.

[Kubuxu]

Closing given #166