Fix a leak with pop in pending_queue

[philipch07]

Description

A leak was found in pop where pendingBaseQueue.pop() would have the slice keep a reference to the popped element.

I also added a regression test to prevent us from having this issue again.

I am unsure if we still want to turn this into a list to allow for gc as referenced in here: #264 considering that we still want the v2 rewrite. I don't mind looking into rebasing that PR if there's any demand for it.

Reference issue

The issue was reported here: #264 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.12%. Comparing base (c875185) to head (cf65cf5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   81.77%   82.12%   +0.34%     
==========================================
  Files          51       51              
  Lines        4522     4525       +3     
==========================================
+ Hits         3698     3716      +18     
+ Misses        680      670      -10     
+ Partials      144      139       -5     

Flag	Coverage Δ
go	82.12% <100.00%> (+0.34%)	⬆️

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