Add Put() to graphdriver API and use it

This makes all users of Put() have a corresponding call
to Get() which means we will be able to track whether
any particular ID is in use and if not unmount it.
devicemapper: Unmount inactive devices

This implements the new Put() operation such that
Get()/Put() maintains a refcount for each ID, mounting
only on first Get() and unmounting on the last Get().

This means we avoid littering the system with lots of mounts
and active devicemapper devices and free resources related
to them.
Remove container.EnsureMounted

This was deprecated already and all it did was call Mount().
The use of this was a bit confusing since we need to pair Mount/Unmount
calls which wasn't obvious with "EnsureMounted".