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/policy.go

@@ -69,7 +69,7 @@ func (sys *PolicySys) IsAllowed(args policy.Args) bool {
 	if globalIsGateway {
 		// When gateway is enabled, no cached value
 		// is used to validate bucket policies.
-		objAPI := globalObjectAPI
+		objAPI := newObjectLayerFn()
 		if objAPI != nil {
 			config, err := objAPI.GetBucketPolicy(context.Background(), args.BucketName)
 			if err == nil {