JIRA: https://jira.spring.io/browse/INT-3897
The `StompMessageHandler` synchronized on the `sessionHandler` during `connectIfNecessary()`
performing mutually exclusive connect operation in the `StompSessionManager`.
Since the last one synchronizes on all `sessionHandler`s during its own operations, we end up with the deadlock
waiting for the `connectSemaphore`.
Change synchronization monitor object to the `connectSemaphore` accomplishing the same desired mutually exclusive connect logic
in the `StompMessageHandler`.
**Cherry-pick to 4.2.x**
c810aaae12