Prevent duplicated User Workspaces when fetching Collections or Favorites for the very first time

As described in NXP-19009, two concurrent requests trying to access the User Workspace for the very first time may lead to the duplication of one's UserWorkspace.

This causes quite some failures in Web UI IT tests :

• collections;from-browser,-add-document-to-a-collection-and-remove-it.And I can see the document belongs to the "MyCollection" collection

  Error Message
  
  Error: Promise was rejected with the following reason: timeout
  Stacktrace
  
  Error: Promise was rejected with the following reason: timeout
      at Browser.hasCollection (test/pages/ui/browser.js:58:12)
      at World.<anonymous> (test/features/step_definitions/document.js:138:23)
      at node_modules/meteor-promise/fiber_pool.js:32:39
  

• favorites;favorites.And I can see the document belongs to the favorites

  Error Message
  
  AssertionError: expected false to be true
  Stacktrace
  
  AssertionError: expected false to be true
      at doAsserterAsyncAndAddThen (/opt/jenkins/workspace/master/plugins_nuxeo-web-ui-master-8.10/nuxeo-web-ui-ftest/webdriver/node_modules/chai-as-promised/lib/chai-as-promised.js:293:29)
      at .<anonymous> (/opt/jenkins/workspace/master/plugins_nuxeo-web-ui-master-8.10/nuxeo-web-ui-ftest/webdriver/node_modules/chai-as-promised/lib/chai-as-promised.js:283:21)
      at Object.defineProperty.get (/opt/jenkins/workspace/master/plugins_nuxeo-web-ui-master-8.10/nuxeo-web-ui-ftest/webdriver/node_modules/chai/lib/chai/utils/overwriteProperty.js:50:37)
      at World.<anonymous> (/opt/jenkins/workspace/master/plugins_nuxeo-web-ui-master-8.10/nuxeo-web-ui-ftest/webdriver/test/features/step_definitions/favorites.js:5:7)
      at /opt/jenkins/workspace/master/plugins_nuxeo-web-ui-master-8.10/nuxeo-web-ui-ftest/webdriver/node_modules/meteor-promise/fiber_pool.js:32:39
  

NXP-19009 might not be done soon and in any case, won't likely be backported to 8.10.

So we need at least, when trying to retrieve Collections or Favorites, lock the parent folder while doing the creation. All this while being careful with transactions of course.