Extend further validation of config values (#8469)

- This PR allows config KVS to be validated properly
  without being affected by ENV overrides, rejects
  invalid values during set operation

- Expands unit tests and refactors the error handling
  for notification targets, returns error instead of
  ignoring targets for invalid KVS

- Does all the prep-work for implementing safe-mode
  style operation for MinIO server, introduces a new
  global variable to toggle safe mode based operations
  NOTE: this PR itself doesn't provide safe mode operations

cmd/signals.go

@@ -55,7 +55,7 @@ func handleSignals() {
 		// send signal to various go-routines that they need to quit.
 		close(GlobalServiceDoneCh)
 
-		if objAPI := newObjectLayerFn(); objAPI != nil {
+		if objAPI := globalObjectAPI; objAPI != nil {
 			oerr = objAPI.Shutdown(context.Background())
 			logger.LogIf(context.Background(), oerr)
 		}
@@ -66,7 +66,7 @@ func handleSignals() {
 	for {
 		select {
 		case err := <-globalHTTPServerErrorCh:
-			if objAPI := newObjectLayerFn(); objAPI != nil {
+			if objAPI := globalObjectAPI; objAPI != nil {
 				objAPI.Shutdown(context.Background())
 			}
 			if err != nil {