reduce getSiteDataFromRecord time from ~300ms to ~2ms

[diracdeltas]

Fix #10273 by minimizing the number of immutableJS to/from native object conversions. The bulk of the performance gain is from removing the line record = Immutable.fromJS(record).

Test Plan:

1. import 6000 bookmarks and wait for them to upload in pyramid 0
2. open pyramid 1, sync to pyramid 0. bookmarks should import within a couple seconds instead of never.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #10281 into master will decrease coverage by 0.02%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master   #10281      +/-   ##
==========================================
- Coverage   53.42%    53.4%   -0.03%     
==========================================
  Files         238      238              
  Lines       21056    21069      +13     
  Branches     3253     3255       +2     
==========================================
+ Hits        11249    11251       +2     
- Misses       9807     9818      +11

Flag	Coverage Δ
#unittest	53.4% <40%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
js/state/syncUtil.js	39.76% <40%> (-0.64%)	⬇️

[diracdeltas]

going to move to 0.21.x so to avoid having to do the work for this PR twice (once for pre-sites-split, one for after)

[diracdeltas]

getting an error upon uploading bookmarks after the rebase:

Error: objectId must be an Array
    at getObjectById (/Users/yan/repos/browser-laptop/js/state/syncUtil.js:453:11)
    at getFolderIdByObjectId (/Users/yan/repos/browser-laptop/js/state/syncUtil.js:491:17)
    at getBookmarkSiteDataFromRecord (/Users/yan/repos/browser-laptop/js/state/syncUtil.js:127:7)
    at Object.module.exports.getSiteDataFromRecord (/Users/yan/repos/browser-laptop/js/state/syncUtil.js:169:17)
    at records.forEach (/Users/yan/repos/browser-laptop/js/state/syncUtil.js:309:39)
    at Array.forEach (<anonymous>)
    at Object.module.exports.applySyncRecords (/Users/yan/repos/browser-laptop/js/state/syncUtil.js:304:11)
    at EventEmitter.ipcMain.on (/Users/yan/repos/browser-laptop/app/sync.js:524:14)
    at emitThree (events.js:116:13)
    at EventEmitter.emit (events.js:197:7)