We store everything we need to index the mempool in the `Mempool` struct.
Currently, we redundantly store all the mempool txs again, but this reduces implementation complexity. Once RAM usage becomes an issue, we can make sure we re-use the node's mempool.
To get a mempool tx, we add `bridge_tx`, which finds the spent coins for us and builds a ffi::Tx.
`make_chronik_block` now sets the time_first_seen of the mempool.