Bring in safe mode support (#8478)

This PR refactors object layer handling such
that upon failure in sub-system initialization
server reaches a stage of safe-mode operation
wherein only certain API operations are enabled
and available.

This allows for fixing many scenarios such as

 - incorrect configuration in vault, etcd,
   notification targets
 - missing files, incomplete config migrations
   unable to read encrypted content etc
 - any other issues related to notification,
   policies, lifecycle etc

cmd/web-router.go

@@ -62,18 +62,8 @@ const specialAssets = "index_bundle.*.js|loader.css|logo.svg|firefox.png|safari.
 func registerWebRouter(router *mux.Router) error {
 	// Initialize Web.
 	web := &webAPIHandlers{
-		ObjectAPI: func() ObjectLayer {
-			if !globalSafeMode {
-				return globalObjectAPI
-			}
-			return nil
-		},
-		CacheAPI: func() CacheObjectLayer {
-			if !globalSafeMode {
-				return globalCacheObjectAPI
-			}
-			return nil
-		},
+		ObjectAPI: newObjectLayerFn,
+		CacheAPI:  newCachedObjectLayerFn,
 	}
 
 	// Initialize a new json2 codec.