dash: decode and encode CbTx v4 (merkleRootAssetUnlocks) #1129

Part of #1047.

CbTx v4 adds merkleRootAssetUnlocks (uint256) after creditPoolBalance. Read and write the new field and include it in the payload size. Today a v4 coinbase leaves 32 bytes unread, which breaks the decoding of every block from v24 activation onwards.