network: refactor networking mesh maintaining code

[algorandskiy]

Summary

This PR refactors meshing code by generalizing and moving meshThread code into a new mesh.go source file and introducing a new mesher abstraction.

By using this abstraction it is now possible to play with mesh creation logic for hybrid network. For example, this PR adds a new hybridRelayMesh that does not run p2p part meshing and just uses ws net meshing.

Additionally this PR changes NewWebsocketNetwork / NewP2PNetwork constructors by introducing GenesisInfo in order to keep a number of parameters manageable.

Test Plan

1. Existing unit tests to confirm the refactoring is OK
2. Added a new unit test to make sure the hybridRelayMesh works as designed

[codecov]

Codecov Report

Attention: Patch coverage is 75.94502% with 70 lines in your changes missing coverage. Please review.

Project coverage is 47.81%. Comparing base (c0c0235) to head (5fefef4).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
network/mesh.go	77.93%	23 Missing and 9 partials ⚠️
network/p2pNetwork.go	59.09%	16 Missing and 2 partials ⚠️
network/wsNetwork.go	77.19%	8 Missing and 5 partials ⚠️
network/hybridNetwork.go	78.12%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6374      +/-   ##
==========================================
- Coverage   50.63%   47.81%   -2.82%     
==========================================
  Files         660      654       -6     
  Lines      110513   110584      +71     
==========================================
- Hits        55953    52875    -3078     
- Misses      51704    54689    +2985     
- Partials     2856     3020     +164     

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[gmalouf]

Looks good so far, left a few comments/questions.

[algorandskiy]

I rewrote the hybrid strategy initialization as suggested, no ugly dummyCreator anymore so it looks cleaner.

[algorandskiy]

Pushed more CR fixes