Use wkspawn from ConcurrentUtilities instead of Threads.spawn

[quinnj]

Should fix #336.

For more context, see the same fix we made for this in CSV.jl.

Basically, objects interpolated into or returned from spawned tasks can be unexpectedly kept alive long after the task has finished and the object should have been garbage-collected due to individual threads holding the most recent task as a reference. Using @wkspawn ensures tasks themselves don't hold references to any of these once they're done executing.

[codecov-commenter]

Codecov Report

Merging #469 (7052a7e) into main (fc7cc2d) will decrease coverage by 0.07%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #469      +/-   ##
==========================================
- Coverage   87.56%   87.50%   -0.07%     
==========================================
  Files          26       26              
  Lines        3280     3280              
==========================================
- Hits         2872     2870       -2     
- Misses        408      410       +2     

Impacted Files	Coverage Δ
src/append.jl	91.01% <50.00%> (-1.13%)	:arrow_down:
src/write.jl	97.56% <50.00%> (-0.31%)	:arrow_down:
src/table.jl	92.49% <100.00%> (ø)

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