Changing user's email: keep using old email before verification new one

[Nes-si]

It's a compliant PR to main PR: parse-community/parse-server#5147. It contains new User method requestEmailChange (w parameter email) that realizes email changing according to new scheme.

[codecov]

Codecov Report

Merging #688 into master will decrease coverage by 5.21%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
- Coverage   90.57%   85.36%   -5.22%     
==========================================
  Files          54       48       -6     
  Lines        4903     3895    -1008     
  Branches     1088      886     -202     
==========================================
- Hits         4441     3325    -1116     
- Misses        462      570     +108

Impacted Files	Coverage Δ
src/ParseUser.js	65.67% <0%> (-16.59%)	⬇️
src/ParsePolygon.js	0% <0%> (-69.5%)	⬇️
src/ParseLiveQuery.js	79.74% <0%> (-20.26%)	⬇️
src/StorageController.react-native.js	84.21% <0%> (-15.79%)	⬇️
src/equals.js	92% <0%> (-8%)	⬇️
src/ParseConfig.js	84.37% <0%> (-3.58%)	⬇️
src/ParseFile.js	94.28% <0%> (-3.05%)	⬇️
src/Parse.js	80.59% <0%> (-2.9%)	⬇️
src/ParseObject.js	88.48% <0%> (-1.54%)	⬇️
... and 26 more

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