Aleth doesn't clear peer's known transactions

[halfalicious]

Aleth maintains a list of transaction hashes for each peer which represent the transactions that the peer knows about - these transactions were either received from the peer (via a Transactions packet) or already sent to the peer. However, Aleth never clears these hashes which means that the tx list can grow unbounded.

Aleth marks a transaction as known for a peer via EthereumPeer::markTransactionAsKnown: https://github.com/ethereum/aleth/blob/4946ed615b11033db61ec47b2a97ef36bd3c047f/libethereum/EthereumPeer.h#L55