txdb: refactor balance.
txdb: refactor balance.

decentraland/bronzeage-node,VidaID/bcoin,VidaID/bcoin,decentraland/bronzeage-node,decentraland/decentraland-node,decentraland/decentraland-node,decentraland/decentraland-node,VidaID/bcoin,decentraland/bronzeage-node