Define and implement layer info cache

This changeset defines the interface for layer info caches. Layer info caches
speed up access to layer meta data accessed in storage driver backends. The
two main operations are tests for repository membership and resolving path and
size information for backend blobs.

Two implementations are available. The main implementation leverages redis to
store layer info. An alternative implementation simply caches layer info in
maps, which should speed up resolution for less sophisticated implementations.

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

docs/storage/cache/memory_test.go

@@ -0,0 +1,9 @@
+package cache
+
+import "testing"
+
+// TestInMemoryLayerInfoCache checks the in memory implementation is working
+// correctly.
+func TestInMemoryLayerInfoCache(t *testing.T) {
+	checkLayerInfoCache(t, NewInMemoryLayerInfoCache())
+}