Improved database connection management

[jcass77]

Resolves #145.

[codecov]

Codecov Report

Merging #148 (12d61e2) into develop (a5ab320) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #148      +/-   ##
===========================================
+ Coverage    97.85%   98.05%   +0.19%     
===========================================
  Files            6        6              
  Lines          326      359      +33     
===========================================
+ Hits           319      352      +33     
  Misses           7        7              

Flag	Coverage Δ
unittests	98.05% <100.00%> (+0.19%)	⬆️

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Impacted Files	Coverage Δ
django_apscheduler/jobstores.py	98.73% <100.00%> (+0.06%)	⬆️
django_apscheduler/models.py	97.01% <100.00%> (+0.04%)	⬆️
django_apscheduler/util.py	97.82% <100.00%> (+2.37%)	⬆️

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