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/gateway-startup-msg.go

@@ -28,8 +28,9 @@ import (
 func printGatewayStartupMessage(apiEndPoints []string, backendType string) {
 	strippedAPIEndpoints := stripStandardPorts(apiEndPoints)
 	// If cache layer is enabled, print cache capacity.
-	if globalCacheObjectAPI != nil {
-		printCacheStorageInfo(globalCacheObjectAPI.StorageInfo(context.Background()))
+	cacheAPI := newCachedObjectLayerFn()
+	if cacheAPI != nil {
+		printCacheStorageInfo(cacheAPI.StorageInfo(context.Background()))
 	}
 	// Prints credential.
 	printGatewayCommonMsg(strippedAPIEndpoints)