feat: use tx hash instead of nonce to determine if the tx is a mempool tx

[0xdapper]

Using nonce to determine is a rather naive way to compare txs. There can be a different tx with same nonce in the mempool. Searchers could want to send same txs with different gas prices(different tx hashes) to the mempool from their EOAs. They could be capturing different opportunities altogether at same nonce too. Nonce cannot be used in place of a tx hash.