Merge changes I35588ed4,I72a3b4db,I25b9cfd7,I98d5e90c,I6d48c0c6 into sc-dev

* changes:
  Have MediaScanner operate on MediaVolume.
  Start using MediaVolume in attachVolume()/detachVolume().
  Make VolumeCache multi-user aware.
  Add User to LocalCallingIdentity.
  Introduce VolumeCache and MediaVolume.