Publish Pando-compatible metadata over GraphSync

[masih]

Implement a publisher specific to Pando metadata that uses the CID of
RecordUpdate as byte value of Pando metadata and exposes a GraphSync
server that can resolve both Pando metadata, and the RecordUpdate
CIDs.

Decouple statedb from publication by changing the state interface to
return the CID of RecordUpdate. This removes all libp2p and legs code
from the state package. Instead, the publication is done on the
controller itself upon POSTs to /complete/<workedby> as a
non-critical operation. This makes a better structured code in the
context of Separation of Concerns.

Relates to:

• Integrate a go-legs publisher endpoint to publish dealbot metadata #342