Generate _localId as UUID

[dplewis]

Closes: #899, #955

Any advice or a different implementation would be appreciated.

[codecov]

Codecov Report

Merging #956 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #956   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files          54       54           
  Lines        5064     5064           
  Branches     1137     1137           
=======================================
  Hits         4656     4656           
  Misses        408      408

Impacted Files	Coverage Δ
src/ParseObject.js	89.42% <100%> (ø)	⬆️

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[acinader]

this seems exactly right to me, both in being 'more correct' and also solving the issues raised.