pool: track pending cost per payer

[libotony]

Description

Addresses vechain/protocol-board-repo#311

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.30769% with 6 lines in your changes missing coverage. Please review.

Project coverage is 62.65%. Comparing base (fd5e569) to head (5644d7f).

Files with missing lines	Patch %	Lines
runtime/resolved_tx.go	57.14%	3 Missing ⚠️
txpool/tx_pool.go	81.25%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
+ Coverage   62.51%   62.65%   +0.13%     
==========================================
  Files         208      208              
  Lines       18872    18926      +54     
==========================================
+ Hits        11798    11858      +60     
+ Misses       5975     5970       -5     
+ Partials     1099     1098       -1     

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[otherview]

Good stuff. It looks good to me, only that concern around possible no-cost txs ?