fix(net): Reduce maximum number of connections per IP to 1

[teor2345]

Motivation

After running PR #6980 over the weekend, I started getting these logs:

2023-06-18T22:11:23.779916Z INFO crawl: zebra_network::peer_set::set: duplicate IP addresses in peer set duplicate_connections=13 duplicated_peers=6 peers=46

This means that 15% of the peer set (7/46) is redundant connections, which seems like a peer set takeover risk.

I'm running 3 nodes locally on 2 IPs, so it's possible that 4 of those connections are from my local setup. But that would still be 11% redundant connections (5/46) from external nodes.

I suggest we aim for 5-10% redundant connections at most.

In my setup:

• 1 peer per IP would be 0% redundant connections. (Unless a peer has multiple IP addresses, which we can't detect.)
• 2 peers per IP would be 9% redundant connections from external nodes (4/46), or 13% from all nodes (6/46).

I don't mind whether we choose 1 or 2, but 3 peers per IP seems to allow too many redundant connections.

Specifications

zcashd's limit is one connection per IP address:
https://github.com/zcash/zcash/blob/fa3827656df8f97fb4700b22022cc581fbda61fa/src/net.cpp#L1663

Solution

Limit Zebra to 1 connection per IP address.

Related fixes:

• Fix tests that require more than 1 connection per IP

Review

This should go in before the next release.

@arya2 wrote the original PR #6980.

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?

[codecov]

Codecov Report

Merging #6993 (85fda39) into main (73ce8fb) will decrease coverage by 0.18%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6993      +/-   ##
==========================================
- Coverage   77.55%   77.38%   -0.18%     
==========================================
  Files         310      310              
  Lines       41582    41582              
==========================================
- Hits        32248    32177      -71     
- Misses       9334     9405      +71