tx-submission decision logic for the inbound side

[coot]

To smooth traffic and save bandwidth, we can add a decision logic to inbound tx-submission protocol which would decide from whom to download an available transaction and trace which transactions are being downloaded (so we don't start to download something that's already being downloaded).

todo list

• [ ] validate tx size
• [ ] validate that tx size is smaller than maximum tx size
• [ ] efficient adding txs to mempool (do not try add the same tx twice) Is this even needed? Mempool could check txids before doing any expensive validation.