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...
Doh... As usual though :smile: