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/global-heal.go

@@ -156,7 +156,7 @@ func execLeaderTasks(sets *xlSets) {
 func startGlobalHeal() {
 	var objAPI ObjectLayer
 	for {
-		objAPI = globalObjectAPI
+		objAPI = newObjectLayerWithoutSafeModeFn()
 		if objAPI == nil {
 			time.Sleep(time.Second)
 			continue