refactor(p2p): split hathor.p2p.peer_discovery into separate modules

[jansegre]

Motivation

The hathor.p2p.peer_discovery module aggregates the abstraction and implementations, it could be split to make future implementations cleaner to review.

Acceptance Criteria

• Split hathor.p2p.peer_discovery into the following submodules: peer_discovery (abstraction), dns (DNS implementation), bootstrap (bootstrap implementation), expose everything the same way they were being exposed before.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 84.99%. Comparing base (40a25dc) to head (8aafd01).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1055      +/-   ##
==========================================
+ Coverage   84.92%   84.99%   +0.07%     
==========================================
  Files         297      300       +3     
  Lines       22932    22943      +11     
  Branches     3470     3470              
==========================================
+ Hits        19475    19501      +26     
+ Misses       2770     2760      -10     
+ Partials      687      682       -5     

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