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/background-newdisks-heal-ops.go

@@ -36,7 +36,7 @@ func monitorLocalDisksAndHeal() {
 	// Wait until the object layer is ready
 	var objAPI ObjectLayer
 	for {
-		objAPI = newObjectLayerFn()
+		objAPI = globalObjectAPI
 		if objAPI == nil {
 			time.Sleep(time.Second)
 			continue