Use connection validator to ping the server. #2485

* Stop the quota job from having an effect on account status
  as it can sometimes take a long time to reply. See #2485 and
  owncloud/core#12744.

* Instead of indirectly using the quota job, use the connection
  validator to regularly ping the server with a basic PROPFIND for
  'getlastmodified' on /.
  This request was usually fast for users even when the quota PROPFIND
  was slow.