Subscription support

[firatkucuk]

• [x] Provide end-to-end websocket communication
• [x] Version bump for Spring 3.0.10
• [x] Better data generation example for testing multiple server instances and multiple sessions
• [x] Test for sequential connection testing
• [x] Graceful connection close
• [x] Test for multiple sessions
• [x] Simplify subscription handlers
• [x] Implement a DefaultRemoteSubscriptionRetriever
• [x] Check Nullability annotations
• [x] Provide HTML tester page
• [x] Complete architecture documentation
• [x] Method renamings and final refactor
• [x] Add Javadocs comments
• [x] Add Websocket communication for GraphQL documentation