fix(p2p): regression after entrypoint refactor

[jansegre]

Motivation

#1086 introduced a regression as can be seen in the failed docker build, during the image test

Acceptance Criteria

• Entrypoint.peer_id should default to None (mypy didn't catch this)
• defer.gatherResults had to be chained before passing it to set
• ProtocolMessages.PEERS data was being constructed with Entrypoint objects, which can't be used in json.dumps, it needed to be converted to str
• Adjust PeerId.__str__ so it uses str(entrypoint) instead of repr(entrypoint) to keep it less verbose
• Add tests that cover all the lines that have errors

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.10%. Comparing base (eb74c11) to head (835b227).
Report is 1 commits behind head on master.

Files	Patch %	Lines
hathor/p2p/peer_discovery/dns.py	90.47%	2 Missing ⚠️
hathor/p2p/peer_id.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
+ Coverage   84.98%   85.10%   +0.11%     
==========================================
  Files         310      310              
  Lines       23495    23506      +11     
  Branches     3549     3551       +2     
==========================================
+ Hits        19968    20005      +37     
+ Misses       2824     2799      -25     
+ Partials      703      702       -1     

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