zot: run GC after manifest removal

Clients today expect the repo to clean up if there are unused blobs, not to
manually delete things they think are unused. Let's do that, and use
umoci's code to do it since it's tested and works.

v2: also run GC on update as well as delete

v3: fix up error return paths needing two args

Signed-off-by: Serge Hallyn <shallyn@cisco.com>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>

pkg/storage/storage_test.go

@@ -135,8 +135,6 @@ func TestAPIs(t *testing.T) {
 					So(err, ShouldNotBeNil)
 				})
 
-				err = il.DeleteBlob("test", d.String())
-				So(err, ShouldBeNil)
 			})
 
 			err = il.DeleteBlobUpload("test", v)