refactor: create VertexHandler [part 1/2]

[glevco]

Motivation

Move all HathorManager.on_new_tx() related code to its own new class, VertexHandler, improving organization and modularization, and as a prerequisite for the upcoming changes related to multiprocess verification.

Acceptance Criteria

• Move HathorManager.on_new_tx() implementation to new VertexHandler, along with other methods used only by it.
  • Everything should be moved as-is, no behavior should change. The only exception is that the conn and skip_block_weight_verification arguments were removed, since they were not used.
• Keep HathorManager.on_new_tx() with a call to VertexHandler.on_new_tx(), compatibility with current code.
• Update Builder and CliBuilder accordingly.
• Fix TestBuilder._get_reactor() (it was not persisting the newly created reactor instance).

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 88.31169% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 85.00%. Comparing base (d08ff49) to head (13e7a24).

Files	Patch %	Lines
hathor/vertex_handler/vertex_handler.py	85.21%	8 Missing and 9 partials ⚠️
hathor/builder/builder.py	96.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #987      +/-   ##
==========================================
+ Coverage   84.92%   85.00%   +0.08%     
==========================================
  Files         296      298       +2     
  Lines       22875    22925      +50     
  Branches     3460     3463       +3     
==========================================
+ Hits        19426    19488      +62     
+ Misses       2764     2756       -8     
+ Partials      685      681       -4     

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