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/api-router.go

@@ -37,8 +37,18 @@ type objectAPIHandlers struct {
 func registerAPIRouter(router *mux.Router, encryptionEnabled, allowSSEKMS bool) {
 	// Initialize API.
 	api := objectAPIHandlers{
-		ObjectAPI: newObjectLayerFn,
-		CacheAPI:  newCacheObjectsFn,
+		ObjectAPI: func() ObjectLayer {
+			if !globalSafeMode {
+				return globalObjectAPI
+			}
+			return nil
+		},
+		CacheAPI: func() CacheObjectLayer {
+			if !globalSafeMode {
+				return globalCacheObjectAPI
+			}
+			return nil
+		},
 		EncryptionEnabled: func() bool {
 			return encryptionEnabled
 		},