Support multiple ID arguments to ::create_or_update and ::update

For ::create_or_update, it's just a convenience, as any needed updates
will still be executed one at a time. For ::update, though, a multiget
will automatically be used to load the documents initially, which should
give a performance boost when updating a lot of documents at once by
avoiding N GET requests.
::update_or_create method on scopes

Takes one or more IDs, and first tries to load them and perform a
concurrency-safe update them; for any documents not returned, flips it
to a ::create_or_update
Bump version