be sure to unlock objects returned during grouping

[MichaelChirico]

Part of #4159

I'm not sure this is the best way to do this, open to alternatives. But I did it recursively since we can really control how "deep down" any usages of .SD to be returned would be.

[codecov]

Codecov Report

Merging #4160 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4160      +/-   ##
==========================================
+ Coverage   99.54%   99.54%   +<.01%     
==========================================
  Files          72       72              
  Lines       13915    13921       +6     
==========================================
+ Hits        13851    13857       +6     
  Misses         64       64

Impacted Files	Coverage Δ
R/data.table.R	100% <100%> (ø)	⬆️

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