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/healthcheck-handler.go

@@ -52,9 +52,9 @@ func ReadinessCheckHandler(w http.ResponseWriter, r *http.Request) {
 func LivenessCheckHandler(w http.ResponseWriter, r *http.Request) {
 	ctx := newContext(r, w, "LivenessCheckHandler")
 
-	objLayer := newObjectLayerFn()
+	objLayer := globalObjectAPI
 	// Service not initialized yet
-	if objLayer == nil {
+	if objLayer == nil || globalSafeMode {
 		// Respond with 200 OK while server initializes to ensure a distributed cluster
 		// is able to start on orchestration platforms like Docker Swarm.
 		// Refer https://github.com/minio/minio/issues/8140 for more details.