Don't require a DB connection, but release one if it is acquired

[jakeonfire]

solves #1808 which evolved from #1320. tip from https://stackoverflow.com/a/26733385.

[codecov-commenter]

Codecov Report

Merging #1812 (e22dd5d) into master (fb80e1f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1812      +/-   ##
==========================================
- Coverage   98.45%   98.45%   -0.01%     
==========================================
  Files         145      145              
  Lines        8589     8587       -2     
==========================================
- Hits         8456     8454       -2     
  Misses        133      133              

Impacted Files	Coverage Δ
sentry-rails/lib/sentry/rails/background_worker.rb	100.00% <100.00%> (+14.28%)	⬆️
sentry-resque/lib/sentry/resque.rb	97.14% <0.00%> (-2.86%)	⬇️

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[st0012]

Just checked that #with_connection uses #release_connection underneath, so calling it directly is even better. Thanks for the fix 👍