refactor DatabaseBackend

[sergei-iurchenko]

No description provided.

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (bc9707d) 90.21% compared to head (133e685) 92.46%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
constance/backends/database.py	72.72%	6 Missing ⚠️
constance/base.py	80.76%	3 Missing and 2 partials ⚠️
constance/checks.py	70.00%	2 Missing and 1 partial ⚠️
constance/backends/memory.py	0.00%	1 Missing and 1 partial ⚠️
constance/backends/redisd.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
+ Coverage   90.21%   92.46%   +2.24%     
==========================================
  Files          31       31              
  Lines        1053     1022      -31     
  Branches      156      147       -9     
==========================================
- Hits          950      945       -5     
+ Misses         68       49      -19     
+ Partials       35       28       -7     

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[camilonova]

@sergei-iurchenko would you mind to work on this separately and create a PR when ready? (so we can close this)