ListObjects Metadata Caching (#10648)

Design: https://gist.github.com/klauspost/025c09b48ed4a1293c917cecfabdf21c

Gist of improvements:

* Cross-server caching and listing will use the same data across servers and requests.
* Lists can be arbitrarily resumed at a constant speed.
* Metadata for all files scanned is stored for streaming retrieval.
* The existing bloom filters controlled by the crawler is used for validating caches.
* Concurrent requests for the same data (or parts of it) will not spawn additional walkers.
* Listing a subdirectory of an existing recursive cache will use the cache.
* All listing operations are fully streamable so the number of objects in a bucket no 
  longer dictates the amount of memory.
* Listings can be handled by any server within the cluster.
* Caches are cleaned up when out of date or superseded by a more recent one.

cmd/api-router.go

@@ -44,6 +44,12 @@ func newCachedObjectLayerFn() CacheObjectLayer {
 	return globalCacheObjectAPI
 }
 
+func setObjectLayer(o ObjectLayer) {
+	globalObjLayerMutex.Lock()
+	globalObjectAPI = o
+	globalObjLayerMutex.Unlock()
+}
+
 // objectAPIHandler implements and provides http handlers for S3 API.
 type objectAPIHandlers struct {
 	ObjectAPI func() ObjectLayer
@@ -320,7 +326,7 @@ func registerAPIRouter(router *mux.Router) {
 
 	// If none of the routes match add default error handler routes
 	apiRouter.NotFoundHandler = collectAPIStats("notfound", httpTraceAll(errorResponseHandler))
-	apiRouter.MethodNotAllowedHandler = collectAPIStats("methodnotallowed", httpTraceAll(errorResponseHandler))
+	apiRouter.MethodNotAllowedHandler = collectAPIStats("methodnotallowed", httpTraceAll(methodNotAllowedHandler("S3")))
 
 }