na.omit respects missing nanotime values

[MichaelChirico]

Closes #4744

As noted, complete.cases itself cannot be helped since it's a non-S3 stats function written in C & agnostic of bit64's NA value.

Nevertheless, with na.omit users can get the same desired behavior of DT[complete.cases(DT)] with na.omit(DT).

[codecov]

Codecov Report

Merging #4752 (b465007) into master (51144d2) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4752      +/-   ##
==========================================
+ Coverage   99.51%   99.54%   +0.02%     
==========================================
  Files          76       76              
  Lines       14695    14688       -7     
==========================================
- Hits        14624    14621       -3     
+ Misses         71       67       -4     

Impacted Files	Coverage Δ
src/assign.c	100.00% <100.00%> (ø)
src/between.c	100.00% <100.00%> (ø)
src/coalesce.c	100.00% <100.00%> (ø)
src/fifelse.c	100.00% <100.00%> (ø)
src/nafill.c	100.00% <100.00%> (ø)
src/shift.c	100.00% <100.00%> (ø)
src/utils.c	97.52% <100.00%> (+1.53%)	⬆️

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