Use DataToken address as the asset did

[ssallam]

Publishing a dataset:

• Deploy/create a DataToken contract -> dtAddress
  • Leave the blob empty (no changes in the contract)
  • User can still set a blob
• Create did = 'did:op:dtAddress'
• Publish ddo/asset on-chain in the DDO contract
  • Change did type from bytes32 to address
  • The did on-chain is now the dtAddress
  • Checks:
    • did is a valid DataToken address
    • msg.sender is the minter of dtAddress (did)
    • did is not already in the DDO registry