Bridge: don't skip old data in the 'loadAnnotations' notifications

Earlier when a 'loadAnnotations' notification was sent on the bridge plugin,
 * Only locally created annotations were included; annotations with
   existing bridge tags were never sent. This was true even if the
   'annotationsLoaded` event (which we are reacting to) contained such
   annotations.
 * On the receiving end, existing annotations were dropped without being
   parsed.

This made it impossible to propagate data via this notification.
However, when we do the actual anchoring in the host document, we are
trying to use this notification to spread the data which become available
as the result of the anchoring; for example, the visual diffs.

To fix this, I have made the following changes:
 * All annotations (coming from the 'annotationsLoaded' event are sent
   with the notifications
 * On the receiving end, all annotations are parsed, so that the updates
   in various fields can be merged in. Of course the already existing
   annotations are not included in the subsequent `loadAnnotations()` call.

This fixes #807.
Merge pull request #808 from hypothesis/807-fix-bridge-dataflow

Bridge: don't skip old data in the 'loadAnnotations' notifications