Update item access time less often.

Making this fully correct is tricky (starting with "correctness" not being
easy to define for all corner cases such as arbitrarily changing cache
policies), and slow.

So, instead just improve the approximation slightly for now. So far we
updated the access time on each access asking for any kind of payload.
This included for example mail folder listings. Which is slow and avoids
cache expunging for anything in folders you opened recently. Instead, just
update the atime for full payload requests.

This makes cache expunging for online accounts actually behave like one
would expect, and it improves folder listing performance, especially under
high I/O load.