fix(network): Limit number of peer connections per IP address, Ignore new peer connections from the same IP and port

[arya2]

Motivation

This PR drops additional peer connections from a given IP after there are already MAX_CONNS_PER_IP in the peer set.

Closes #6936.

Solution

• Define MAX_CONNS_PER_IP
• Count the number of keys in PeerSet.ready_services and PeerSet.cancel_handles before inserting new connections into the peer set
• Drop new connections when there are already MAX_CONNS_PER_IP

Review

This is a routine security fix.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

Follow Up Work

• Ignore repeated inbound connections from the same IP #6981
• Don't initiate handshakes to outbound IPs that were recently connected #6982

[teor2345]

I think this change needs some tests, what do you think?

[codecov]

Codecov Report

Merging #6980 (703a653) into main (484f3d7) will increase coverage by 0.02%.
The diff coverage is 93.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6980      +/-   ##
==========================================
+ Coverage   77.50%   77.52%   +0.02%     
==========================================
  Files         310      310              
  Lines       41534    41582      +48     
==========================================
+ Hits        32192    32238      +46     
- Misses       9342     9344       +2     

[teor2345]

Thanks for the tests!