Return AbstractSubscribableChannel from @Bean methods

Declare SubscribableChannel @Beans in
WebSocketMessageBrokerConfigurationSupport as
AbstractSubscribableChannel to avoid the need for casting when
registering interceptors.

Issue: SPR-11065
Simplify determination of SockJS path

The SockJS path is now passed to the SockJsService handleRequest method
thus removing the need to guess it.

Issue: SPR-11058