Add garbage collection as a background process and policy
configuration for configuring when to run garbage collection.
By default garbage collection will run when deletion occurs
and no more than 20ms out of every second.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>
Marks and sweeps unreferenced objects.
Add snapshot cleanup to metadata.
Add content garbage collection
Add dirty flags for snapshotters and content store which
are set on deletion and used during the next garbage collection.
Cleanup content store backend when content metadata is removed.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>
