L1 P2P: avoid repeated GetPooledTransactions requests

[Arkenan]

Low priority. Right now when a PooledTransactionHashes message arrives to our node we immediately check which transactions we don't have and send a GetPooledTransactions message. There is a possible edge case where we might receive the same pooled transaction hash from multiple peers and perform the request twice. This is not high priority as it would not cause inconsistencies on our node, but it may occupy more bandwidth. It might be useful to keep track of which transactions we're already requesting so we don't request the same one multiple times.