fix(subscription)!: decouple retainAckedMessages from messageRetentionDuration

[callmehiphop]

Fixes #623

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #625 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
- Coverage   97.89%   97.89%   -0.01%     
==========================================
  Files          14       14              
  Lines         857      856       -1     
  Branches      178      178              
==========================================
- Hits          839      838       -1     
  Misses          2        2              
  Partials       16       16

Impacted Files	Coverage Δ
src/subscription.ts	99.18% <ø> (-0.01%)	⬇️

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