INT-1198: WebSockets: Server Side

JIRA: https://jira.spring.io/browse/INT-1198

**Do not merge yet**.
Just for review right now. 
There might be need more tests. The namespace support can be added here too.
Maybe it will have to think about more components like `SubscriptionRegistry` to store `user/subscription` pairs...
There are some volnurabilities from Spring WebSockets here:
- it allows only single `Broker` for entire application. Strange.
- it doesn't provide real `id` for `AbstractBrokerMessageHandler` bean from xml: `MessageBrokerBeanDefinitionParser#registerMessageBroker`
- it populates two beans from annotation config: `AbstractMessageBrokerConfiguration` - `simpleBrokerMessageHandler()` and `stompBrokerRelayMessageHandler()`. And one of them in the end is always some fake bean - `noopBroker`.

Another inconvenience for us, that `Broker` can't be configured without StompEndpoint...

CC @rstoyanchev