Improved performance on rendering 'day' with highlighted days

[alex-shamshurin]

Highlight dates with custom class names and ranges, like this:

 { "react-datepicker__day--highlighted-custom-1": [
      moment().subtract(4, "days"),
      moment().subtract(3, "days"),
      moment().subtract(2, "days"),
      moment().subtract(1, "days") ]
    },
    { "react-datepicker__day--highlighted-custom-2": [
      moment().add(1, "days"),
      moment().add(2, "days"),
      moment().add(3, "days"),
      moment().add(4, "days") ]
    }

were calculated each time on day rendering and that caused slow performance with long arrays.

Now this array of objects is calculated just once in costructor and is kept in state as javascript flat Map of {dateString: className}

[codecov]

Codecov Report

Merging #1180 into master will increase coverage by 0.47%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master    #1180      +/-   ##
==========================================
+ Coverage   86.38%   86.86%   +0.47%     
==========================================
  Files          13       13              
  Lines         852      860       +8     
  Branches      143      139       -4     
==========================================
+ Hits          736      747      +11     
+ Misses         31       30       -1     
+ Partials       85       83       -2

Impacted Files	Coverage Δ
src/calendar.jsx	85.36% <ø> (+1.11%)	⬆️
src/month.jsx	83.72% <ø> (ø)	⬆️
src/week.jsx	77.41% <ø> (ø)	⬆️
src/day.jsx	93.33% <100%> (+2.85%)	⬆️
src/date_utils.js	92.96% <100%> (+0.91%)	⬆️
src/index.jsx	89.72% <75%> (-1.18%)	⬇️

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