Topic-aware consensus-gossip pruning

[rphmeier]

Related to #1645

Consensus gossip (network/consensus_gossip.rs) should maintain an LRU cache of "known dead" topics. There should be a collect_garbage variant specifically for removing all messages on a specific topic.

received messages for a dead topic should be rejected.
All current uses of collect_garbage should be amended where possible to use the new variant.