Remove WebSocketStats in favor of built-in one

The collection of internal stats and counters is now built-in:
https://github.com/spring-projects/spring-framework/commit/ab4864da2a136fbdcdb9af9b4e043ae769652b06

This change removes the hand-rolled one in the sample.
Add SockJsClient to load tests

The StompWebSocketLoadTestClient StompWebSocketLoadTestServer now use
SockJS based on the SockJsClient now available in the Spring Framework.
Catch up to the latest snapshot
Logging related (minor) updates