refactor(vertex-handler): modularize methods [part 2/2]

[glevco]

Depends on https://github.com/HathorNetwork/hathor-core/pull/987

Motivation

Modularize on_new_tx() as a prerequisite for the upcoming changes related to multiprocess verification, in order to create a new aysnc on_new_tx() (we should be able to duplicate the method without copying all code).

Acceptance Criteria

• Rename VertexHandler.on_new_tx() to on_new_vertex().
• Break on_new_vertex() into modularized methods, _validate_vertex(), _save_and_run_consensus(), and _post_consensus() (renamed from tx_fully_validated()).
• No behavior should change, only a couple unnecessary assertions were removed.

Checklist

• [x] 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[bot]]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 84.90%. Comparing base (5d3b757) to head (e09d336).

Files	Patch %	Lines
hathor/vertex_handler/vertex_handler.py	92.30%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #988      +/-   ##
==========================================
- Coverage   84.94%   84.90%   -0.05%     
==========================================
  Files         298      298              
  Lines       22928    22932       +4     
  Branches     3464     3465       +1     
==========================================
- Hits        19476    19470       -6     
- Misses       2767     2772       +5     
- Partials      685      690       +5     

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