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/admin-handlers_test.go

@@ -90,11 +90,7 @@ func prepareAdminXLTestBed() (*adminXLTestBed, error) {
 	globalPolicySys = NewPolicySys()
 	globalPolicySys.Init(buckets, objLayer)
 
-	globalNotificationSys, err = NewNotificationSys(globalServerConfig, globalEndpoints)
-	if err != nil {
-		return nil, err
-	}
-
+	globalNotificationSys = NewNotificationSys(globalEndpoints)
 	globalNotificationSys.Init(buckets, objLayer)
 
 	// Setup admin mgmt REST API handlers.