LDS: Improve pinning unsaved objects

[dplewis]

Closes: #792

Saves _localId into the LocalDatastore to keep consistency when querying.

[codecov]

Codecov Report

Merging #795 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #795      +/-   ##
==========================================
+ Coverage   92.01%   92.02%   +0.01%     
==========================================
  Files          54       54              
  Lines        4910     4917       +7     
  Branches     1088     1091       +3     
==========================================
+ Hits         4518     4525       +7     
  Misses        392      392

Impacted Files	Coverage Δ
src/ParseQuery.js	96.04% <100%> (+0.01%)	⬆️
src/LocalDatastore.js	98.69% <100%> (+0.02%)	⬆️

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[dplewis]

@acinader This PR allows you to updated pinned objects that aren't saved. We find unsaved object by _localId.

[acinader]

this looks good to me.

package-lock.json is dirty? can you revert?

[dplewis]

@acinader It I revert package-lock.json the tests fail travis-ci

Edit: I'll create a separate PR for the package-lock.json