Refactor DeleteExpired for safer behavior

[AxelPrel]

• make sure that expManager queue is not empty before popping, which can happen if an explicit delete happens while DeleteExpired releases the lock.

• evict() now returns shouldContinue instead of a shouldBreak to simplify the loop.

[AxelPrel]

it might also fix #59

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (2601bb0) to head (a613de3).

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Additional details and impacted files

@@            Coverage Diff            @@
##              main       #60   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          527       523    -4     
=========================================
- Hits           527       523    -4     

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[gonejack]

I don't think this would fix #59 as there are no limitation or scale down operations on capacity of expirationManager.queue or expirationManager.map