Fix a race condition that could occur when the client tried to anchor
annotations in a PDF while the PDF was still loading.

PDF.js' `PDFViewer` class initializes its `PDFPageView` objects, as
returned by `PDFViewer.getPageView`, asynchronously. While the page
views are being initialized, `PDFViewer.pagesCount` returns 0 and
`PDFViewer.getPageView` will return a nullish value. When loading has
progressed further, `getPageView` returns a `PDFPageView` but with no
`pdfPage` property. Once the page views are fully ready, a bubbling
"pagesloaded" event is dispatched at the PDF viewer's container DOM
element.

The PDF anchoring code previously assumed that page views were always
immediately available and they always had a `pdfPage` property. If this
was not the case, anchoring would fail and all/many annotations would
appear as orphans in the client. See the associated issue for ways to
reproduce this locally with Via.

Resolve the issue by making the `getPageView` helper which wraps
`PDFViewer.getPageView` async, and block in that function until the page
views are fully ready.

 - Make `getPageView` helper async and block if necessary until page
   views are ready.
 - Add a test to simulate scenarios where page views are not ready yet
 - Change the `quotePositionCache` cache to store page indexes rather
   than `PDFPageView` objects. This means that less code in the file
   needs to know about `PDFPageView` objects, and deal with the fact
   that they may not be immediately available. It also means it should
   be easier to adapt the code to future PDF.js API changes.

Fixes #1330