Bug 51156: Don't subclass HTMLCacheUpdate any more

This fixes template and file updates for templates / files that are used in
more than 10 pages. Tested locally with templates used 20+ times.

TODO: Subclass from generic range splitting base class once that is available
instead of duplicating the range splitting functionality from HTMLCacheUpdate.

Change-Id: I4d74b4b007add0758f726c3311367f9853868aeb
Merge "Bug 51156: Don't subclass HTMLCacheUpdate any more"