package daemon: Initial old packages state support

* VolumeState: Move locking to Volume.
* Package: Pull the file related functionality into new class
  PackageFile. A Package refers to a PackageFile. A PackageFile can be
  referenced by multiple Packages. PackageFiles are managed by the new
  PackageFileManager.
* CommitTransactionHandler: Clone the passed volume state.
* Volume now manages two VolumeStates: A state reflecting the currently
  active packages and the latest state (i.e. the one reflecting the
  situation of the packages directory). Usually the two are the same,
  unless an old state has been booted.
* The client interface hasn't been adjusted yet. Clients only see the
  latest state.
package daemon: Don't apply system package changes immediately

When a system package is going to be deactivated, activate/deactivate
the packages of the whole transaction only to the latest state.
Afterward latest state and active state will differ.