The boltdb image store now manages its own transactions when
one is not provided, but allows the caller to pass in a
transaction through the context. This makes the image store
more similar to the content and snapshot stores. Additionally,
use the reference to the metadata database to mark the content
store as dirty after an image has been deleted. The deletion
of an image means a reference to a piece of content is gone
and therefore garbage collection should be run to check if
any resources can be cleaned up as a result.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>
