Profiling the test case in #556 showed significant overhead from the
processing involved in or triggered by each `UPDATE_ANCHOR_STATUS`
action handled by the store. Previously one `UPDATE_ANCHOR_STATUS`
action was dispatched for each annotation whose anchoring status
changed.

Improve the situation by making the `UPDATE_ANCHOR_STATUS` action handle
updates for multiple annotations at once and modifying the `frameSync`
service to coalesce anchoring status updates from the host page into
a smaller number of `UPDATE_ANCHOR_STATUS` actions.

Fixes #556