Replace dead(not missing) peers & candidates

[aarshkshah1992]

Based on some good ideas by @aschmahmann , here's a couple of things we can do to improve how we manage the replacement cache & "dead" peers:

1. Since the replacement cache is bounded, we could lose potential candidates if it's full. So, when we eject a candidate from the replacement cache either because it goes into the RT or it's not reachable, we should replace it with a connected peer that is neither in the RT nor in the replacement cache.
   See RT connectivity changes go-libp2p-kad-dht#482 (comment) for the motivating discussion.

2. When DHT calls HandleDeadPeer on the RT for a peer, we immediately eject it from the RT without replacing it. Currently, the DHT does this for a connected peer that stops being a DHT server. We should replace such peers.

[aarshkshah1992]

ping @Stebalien.

[aarshkshah1992]

We now have a new RT management algo at #66.