Fix configuration handling bugs: (#5473)

* Update the GetConfig admin API to use the latest version of
  configuration, along with fixes to the corresponding RPCs.
* Remove mutex inside the configuration struct, and inside
  notification struct.
* Use global config mutex where needed.
* Add `serverConfig.ConfigDiff()` that provides a more granular diff
  of what is different between two configurations.

cmd/web-handlers.go

@@ -423,6 +423,10 @@ func (web *webAPIHandlers) SetAuth(r *http.Request, args *SetAuthArgs, reply *Se
 		return toJSONError(err)
 	}
 
+	// Acquire lock before updating global configuration.
+	globalServerConfigMu.Lock()
+	defer globalServerConfigMu.Unlock()
+
 	// Notify all other Minio peers to update credentials
 	errsMap := updateCredsOnPeers(creds)