Make interwiki transclusion use the WAN cache

Make interwiki transclusion use the WAN cache

This means that now:
* Entries actually get deleted when expired
* The transclusion cache is shared across wikis
* Large blobs that do not fit in cache no longer cause DB errors
* DB writes are not triggered on GET requests
* Keys are hashed and no longer need to be so restrictive

Also, add a "check key" based purge system and process cache the
text/html values similar to how regular revision text is cached.

Bug: T189702
Change-Id: I8ac12b53c02bb26857175dd5a4af29d49e03dc33