Synchronously track URIs in the process of opening

[smashwilson]

Synchronously track a set of incoming URIs within a Workspace. If another item is being opened at the same URI, await its Promise before continuing, to ensure that ::open() has a chance to find the Item added by the first call if it succeeds.

Fixes #13914.

[smashwilson]

Hmm, that CircleCI failure might be legit:

ResultsView
  opening results
    it opens the file containing the result in a non-pending state when the search result is double-clicked
      Expected <TextEditor 1552> to be null.
        Error: Expected <TextEditor 1552> to be null.
        at /Users/distiller/atom/node_modules/find-and-replace/spec/results-view-spec.js:417:75
        at Generator.next (<anonymous>)
        at step (/Users/distiller/atom/node_modules/find-and-replace/spec/results-view-spec.js:1:428)
        at <anonymous>

[smashwilson]

atom/find-and-replace#1006 should get this green again. I'll crunch on a few issues over there to... conserve release numbers I guess?

[matthewwithanm]

thanks @smashwilson!