Remove incorrect package import

Issue: SPR-10880
Polish WebSocket/STOMP Java config

Ensure configuration provided for WebSocketHandler's (eg interceptors,
or HandshakeHandler) are passed on to the SockJsService if congiured.

Better separate Servlet-specific parts of the configuration to make it
more obvious where non-Servlet alternatives could fit in.

Add more tests.

Improve WebSocket integration tests.
Add Tomcat WebSocket integration tests
Polish WebSocket integration tests