kvserver: allow configuring the max gossip peers

**Is your feature request related to a problem? Please describe.**
In large clusters we have seen that the gossip network can enter a partially broken state where the "bootstrap set" and many of its direct peers are saturate from both an incoming and outgoing connection perspective. This makes it difficult for new nodes to join the cluster and means that the cluster doesn't have a good "shape" once its stabilized.

**Describe the solution you'd like**
One possible solution is to configure the max peers for the incoming and outgoing connection counts independently. The incoming count could be larger by the outgoing count by some factor (2?) so that the incoming connections are not often saturated. It is hard to fully understand the implications of this change.

**Describe alternatives you've considered**
An alternative gossip implementation with faster convergence. We would have to do more research to figure out what this should look like.

Jira issue: CRDB-39343

Epic CRDB-59090