core: filter out txs with invalid signatures as soon as possible

[duanhao0814]

Once we detect an invalid transaction during recovering signatures, we should
directly exclude this transaction to avoid validating the signatures hereafter.

This should optimize the validations times of transactions with invalid signatures
to only one time.

[karalabe]

I don't really think this is worth the complexity. Transactions with invalid signatures do not get propagated across the network. At the worse a malicious sender is able to put some strain on the peers it's directly connected to, but not on anyone else.

[duanhao0814]

Yes, I agree that transactions with invalid signatures will not get propagated across the network.

And I saw that we need to filter out duplicate transactions before acquire txpool lock and add into txpool, so I thought it's worth to filter out all possible invalid transactions.
At the same time, this change can reduce some lock occupation because currently invalid txs can only be detected after we acquired the lock. If we can filter out them before we try to add it into txpool, we can reduce some competition on the lock of txpool although I'm not sure if it is worth to do it.
So, if this change doesn't make much sense, I will close this PR.

[holiman]

I think this makes sense -- it does the same amount of work as previously, but actually handles a possible error correctly too. I don't think it adds complexity