We add transactions to the DB as new blocks are coming in using TxWriter. Until we index the mempool, we set the time_first_seen to 0.
/tx/:txid returns txs by txid from the DB using the QueryTxs struct; not from the mempool yet though, which will be in a future diff.
We also add a few new protobuf messages: Tx, OutPoint, TxInput, TxOutput, BlockMetadata and Network.