RevisionStorage::update should also store data in ExternalStore

Although ::update is currently unused (I think), it should also store content
to ExternalStore. It appears that it currently simply updates rev_content,
instead of inserting the data into ES & saving the url.

While this works (there will be no "external" flag, so it will just decompress
fine from flow_revision as it does from ES. While there is no functional
breakage, you probably want all content to be stored in ES though, if you've
set that up ;)

Change-Id: I4f34383f8ae70ead0a56ffa68dd3ab398eb2549a
Merge "RevisionStorage::update should also store data in ExternalStore"