registry/storage/cache/memory: Use LRU cache to bound cache size

Instead of letting the cache grow without bound, use a LRU to impose a
size limit.

The limit is configurable through a new `blobdescriptorsize` config key.

Signed-off-by: Aaron Lehmann <alehmann@netflix.com>

registry/storage/cache/memory/memory_test.go

@@ -9,5 +9,5 @@ import (
 // TestInMemoryBlobInfoCache checks the in memory implementation is working
 // correctly.
 func TestInMemoryBlobInfoCache(t *testing.T) {
-	cachecheck.CheckBlobDescriptorCache(t, NewInMemoryBlobDescriptorCacheProvider())
+	cachecheck.CheckBlobDescriptorCache(t, NewInMemoryBlobDescriptorCacheProvider(UnlimitedSize))
 }