https://build.spring.io/browse/INT-MJATS41-1150
When we add `addHandler()` to the `CompositeStompSessionHandler`,
there is no guarantee that we will have `session` atomically during
this method invocation or after it relying on the later call to the
`this.delegates`.
In other words the session may be populated in between and, therefore,
our `delegate` loses `afterConnected()` event.
We need to synchronize on the barrier and block the concurrent
`afterConnected()` or wait for it.
This way we atomically ensure that our `delegate` is added to the
existing `session` or will be performed afterwards in the `afterConnected()`
**Cherry-pick to 4.3.x**
df109410c3