Fix effect cleanups for react 17

[anajavi]

what?

In React 17 the effect cleanup functions run asynchronously..

This PR changes the cleanup functions so, that they should work in React 17 too.

why?

React 17 should release this week.

[codecov-commenter]

Codecov Report

Merging #310 into master will increase coverage by 16.78%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #310       +/-   ##
===========================================
+ Coverage   71.48%   88.27%   +16.78%     
===========================================
  Files          18       82       +64     
  Lines         249     1049      +800     
  Branches       45      206      +161     
===========================================
+ Hits          178      926      +748     
- Misses         63      107       +44     
- Partials        8       16        +8     

Impacted Files	Coverage Δ
...s/react-jsx-highcharts/src/components/Axis/Axis.js	92.85% <100.00%> (ø)
...x-highcharts/src/components/BaseChart/BaseChart.js	97.77% <100.00%> (ø)
...x-highcharts/src/components/ColorAxis/ColorAxis.js	100.00% <100.00%> (ø)
...act-jsx-highcharts/src/components/Series/Series.js	97.10% <100.00%> (ø)
...highcharts/src/components/PlotBandLine/PlotBand.js	100.00% <0.00%> (ø)
packages/react-jsx-highcharts/src/index.js	100.00% <0.00%> (ø)
...arts/src/components/Series/createProvidedSeries.js	100.00% <0.00%> (ø)
...s/src/components/PlotBandLine/PlotBandLineLabel.js	100.00% <0.00%> (ø)
...x-highcharts/src/components/SeriesContext/index.js	100.00% <0.00%> (ø)
... and 58 more

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[whawker]

Great spot on these.

[anajavi]

Great spot on these.

We might have to make unmounting tests async too, but let's see when enzyme lands react 17 support.