refactor(api): prevent duplicate tx earlier

[jansegre]

Motivation

This verification has to happen anyway and doing it earlier avoids wasting time with more expensive verifications.

Acceptance Criteria

• Check for tx existence on HathorManager.push_tx directly before waiting for HathorManger.on_new_tx to do it.

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 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.90%. Comparing base (b20be70) to head (f70cdef).

Files	Patch %	Lines
hathor/manager.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1006      +/-   ##
==========================================
+ Coverage   84.87%   84.90%   +0.03%     
==========================================
  Files         300      300              
  Lines       22943    22945       +2     
  Branches     3470     3471       +1     
==========================================
+ Hits        19473    19482       +9     
+ Misses       2778     2774       -4     
+ Partials      692      689       -3     

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