fix: remove naive seen_txns cache

[anna-carroll]

Given #31 , it's now possible to remove the naive solution of the seen_txns cache, and only filter transactions by whether they've been included on the rollup or not.

[anna-carroll]

As we can see, at this point the TxPoller struct is an extremely simple wrapper for one function that calls the tx cache endpoint. It might make sense to refactor this, to combine it with the BundlePoller struct into a single TransactionCache struct which has two methods: get_transactions and get_bundles

[prestwich]

straightforward