Skip download and verification if the transaction is already in the mempool or state

[teor2345]

Motivation

In the transaction download stream, we want to skip transactions we've already downloaded or verified.

We want to skip:

• witnessed transaction IDs (WtxId) that are already in the mempool
• witnessed transaction IDs (WtxId) that have already been rejected by the mempool
• mined transaction IDs (transaction::Hash) that have already been committed to the state

We need to check:

• transaction IDs before downloading
• downloaded and PushTransactions before verifying

[mpguerra]

I forgot this one also needs an estimate. Please add your planning poker estimate with ZenHub @conradoplg @dconnolly @jvff @oxarbitrage @teor2345 @upbqdn

Probably just an estimate from @conradoplg is fine since they're the one who has actually worked on it already :)