Decouple redis dependency from blob descriptor cache

Ensure that clients can use the blob descriptor cache provider without needing
the redis package.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/cache/memory/memory_test.go

@@ -0,0 +1,13 @@
+package memory
+
+import (
+	"testing"
+
+	"github.com/docker/distribution/registry/storage/cache"
+)
+
+// TestInMemoryBlobInfoCache checks the in memory implementation is working
+// correctly.
+func TestInMemoryBlobInfoCache(t *testing.T) {
+	cache.CheckBlobDescriptorCache(t, NewInMemoryBlobDescriptorCacheProvider())
+}