Overhaul stats: Refactor code related to metrics persistence

[josecelano]

Refactor code related to metrics persistence.

[josecelano]

ACK bd6e06a

[codecov]

Codecov Report

Attention: Patch coverage is 84.92063% with 19 lines in your changes missing coverage. Please review.

Project coverage is 83.35%. Comparing base (46df7eb) to head (bd6e06a).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ackages/tracker-core/src/databases/driver/mysql.rs	0.00%	9 Missing ⚠️
...tracker-core/src/statistics/persisted/downloads.rs	85.18%	2 Missing and 2 partials ⚠️
...kages/tracker-core/src/statistics/persisted/mod.rs	0.00%	1 Missing and 1 partial ⚠️
src/app.rs	0.00%	2 Missing ⚠️
packages/tracker-core/src/announce_handler.rs	66.66%	0 Missing and 1 partial ⚠️
packages/tracker-core/src/torrent/manager.rs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1542      +/-   ##
===========================================
+ Coverage    83.33%   83.35%   +0.01%     
===========================================
  Files          274      274              
  Lines        19554    19547       -7     
  Branches     19554    19547       -7     
===========================================
- Hits         16295    16293       -2     
+ Misses        2969     2965       -4     
+ Partials       290      289       -1     

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