gc: add a policy to skip garbage collecting new blobs

We perform inline garbage collection of orphan blobs. However, the
dist-spec poses a problem because blobs begin their life as orphan blobs
and then a manifest is add which refers to these blobs.

We use umoci's GC() to perform garbage collection and policy support
has been added recently which can control whether a blob can be skipped
for GC.

In this patch, we use a time-based policy to skip blobs.

pkg/api/BUILD.bazel

@@ -33,7 +33,7 @@ go_library(
 
 go_test(
     name = "go_default_test",
-    timeout = "short",
+    timeout = "moderate",
     srcs = ["controller_test.go"],
     data = [
         "//:exported_testdata",