Get all shares as iterable

Sometimes we need all shares or rather a specific subset of shares but
creating dedicated functions is a pain. This just returns an iterable
object for all shares so we can loop over them without allocating all
the memory on the system.

It should not be used by any user called code. But in an occ command or
background job it is fine IMO.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>