/transaction-info: add creator's pubkey to txindex metadata

[alecov]

In tandem with #297:

In /transaction-info, txindex metadata for both CC/DAO transfers identifies the creator by username instead of pubkey, for example:

"CreatorCoinTransferTxindexMetadata": {
    "CreatorUsername": "BeActive",
    "CreatorCoinToTransferNanos": 10000,
    "DiamondLevel": 0,
    "PostHashHex": ""
}

This is really bad since AFAIK usernames are volatile and, even if they're not in case of already issued CCs/DAOs, it would require a second RPC call in order to grab the name's pubkey.

I would suggest improving the output of /transaction-info by adding the creator's pubkey in the txindex metadata.

[lazynina]

@alecov - I would welcome a PR to add the CreatorPublicKey to these structs as you suggested. You can make this update in core by adding a CreatorPublicKeyBase58Check field to CreatorCoinTransferTxindexMetadata and DAOCoinTransferTxindexMetadata (found in db_utils around line 3588) and then updating the ComputeTransactionMetadata logic in mempool.go in the ComputeTransactionMetadata function