P2P transaction flow is spammy

[Arkenan]

There are separate problems to fix:

Urgent and simple:

• Add deduplication. If we receive the same unknown transaction hash by N different nodes we may send N requests for GetPooledTransactions, when in reality we only need one. To achieve this we need to track in-flight transactions.
• Avoid sending empty requests. If we receive an empty transactions list we still send a message. We shouldn't.

Medium:

• Rate limiting/batching. We probably should wait some milliseconds to accumulate transactions to send a request for all of them in a batch instead of doing this eagerly.
• is_synced guard: if we aren't synced we shouldn't even send the request for the transactions, it's no use. We will not building blocks soon.

Needs more research.

• We should probably not send this request to a random peer but to the sender peer. This we need to check in the specs.
• Are we using the broadcaster and not notifying it? Is it possible that we are sending the hash to the same node that sends it to us?
• Node selection: use the weighted version or some other mechanism, not random.