This PR Removes:

• General:
  • addGraphQLSubscriptions => use apollo-link-split instead. more info
• Server:
  • onSubscribe => use onOperation instead.
  • onUnsubscribe => use onOperationComplete instead.
• Client:
  • onConnect => use onConnecting or onConnected instead.
  • onDisconnect => use onDisconnected instead.
  • onReconnect => use onReconnecting or onReconnected instead.
  • unsubscribe => use Subscription.unsubscribe instead.
  • query => use request instead.
  • subscribe => use request instead.

Also, aligns tests.

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

Related to #261