Updates, fixes, optimization

[vadimka123]

1. Fix exists tests for (fixed action meta):
   • should call onChange with null on hitting backspace when backspaceRemovesValue is true and isMulti is false
   • should call onChange with an array on hitting backspace when backspaceRemovesValue is true and isMulti is true
2. Simplify small part of code
3. Fixed isValidNewOption when value is null
4. Fixed duplicate declaration in tests
5. Very big performance issue: buildMenuOptions only when menu is opened
   I'm have page with ~1000 selects with a ~1000 options. When render this page it takes ~ 40-50s and while page either hangs terribly or crashed. After this update page render takes 10s and there are no hangs. It's very need update.
6. Fix check for ensure focus. Now code compiled as (isFocused && !isDisabled && prevProps.isDisabled || isFocused && menuIsOpen && !prevProps.menuIsOpen). See Removes parentheses in expressions with mixed operators prettier/prettier#187

Fixed next issues:
#3128
#3055
#2970
#2711

[kylealwyn]

Was coming here regarding a similar performance issue where react-select isn't being interacted with yet but is still hogging up the flame graph. Happy to provide profile if necessary

[vadimka123]

@kylealwyn, please, provide profile

[mike391]

@gwyneplaine this is very nice, worth taking a look. Looking forward to having it merged

[jacobworrel]

looking forward to this seeing this merged. the buildMenuOptions is a major bottleneck when dealing with many dropdowns with potentially large lists.

[vadimka123]

@JedWatson, any progress on review ?
It's very need

[piotrwitek]

@vadimka123 thanks, I'm using your fork and looking great, it's a salvation for my project!

Note to others: It looks like it still requires: filterOption={createFilter({ ignoreAccents: false })} to fix perf issues when filtering large options sets.

@JedWatson please could you review this PR? It's solving critical performance issues, very important.

[abenhamdine]

@vadimka123 great work, but IMHO your PR mix too much different concerns, and your changes will have more chances to be merged if you break it down in several PR.
Eg you could first PR only the performance fix https://github.com/vadimka123/react-select/commit/71b8a37f55795188a93b880aa864bbb39aca0c65, it would be easier to review for the maintainers.

[vadimka123]

Changes splited to different PR
#3566
#3567
#3568
#3569

[JedWatson]

That perf fix has been merged with #3569 and will be released in a moment