JIRA: https://jira.spring.io/browse/INT-4271
The `Flusher.run()` is `synchronized` on the `FWMH`; a `stop()` while it's running will interrupt the task
which could cause a state to be removed without a flush actually happening (interrupt
on lock acquisition).
- synchronize `stop()` so it won't interrupt a running flusher
- detect an interrupt in the flusher and stop flushing
**Cherry-pick to 4.3.x**
(cherry picked from commit a38bf60)
92067f10d0