Try to reuse a cached DOM for selective serialization

Instead of re-parsing, use a cache service (if configured) to get the DOM for
the oldid this revision is based on. This will significantly speed up page
saving and avoid the associated API requests.

Change-Id: Ifaf0d9dc8572650d221d3dc1d29770448a61ee93
Merge "Try to reuse a cached DOM for selective serialization"