eth/handler: check for tx on chain as well

[cskiraly]

The fetcher should not fetch transactions that are already on chain. Until now we were only checking in the txpool, but that does not have the old transaction. This was leading to extra fetches of transactions that were annoounced by a peer but are already on chain.

Here we extend the check to the chain as well. Still WIP, as this check might be expensive, and there are other options to do the same check.

[cskiraly]

@rjl493456442 , I've committed this fix for now, but there are other options. Curious which one you would prefer:

• I think the check above is expensive as GetCanonicalTransaction pulls the tx. We can implement a cheaper HasCanonicalTransaction to do the check.
• I also have a variant that is not looking at the chain, but has a "seen" cache directly in the tx_fetcher.

I can also separate the hasTx callback from a chainTx callback, and pass both to the handler. Seems cleaner.
What do you think?