When receiving annotation updates via the WebSocket, merge the updated
annotation with the existing local annotation, preserving the local tag
and anchoring status flags.

This fixes a problem where Annotations would be shown as Orphans after
an update was received via the WebSocket

 * When an annotation update is received, merge the current/new
   versions rather than removing the current version and replacing
   it with the new one.

 * Remove mutation of existing annotation in `loadAnnotations()`,
   since the reducer function is now responsible for merging changes
   and triggering UI updates