INT-3473: Inactive `TcpNioFactory` before stop

JIRA: https://jira.spring.io/browse/INT-3473

The simple fix to prevent `Selector closed` error loging message on `TcpNioServerConnectionFactory#stop()`

No one test falls.

**Cherry-pick to 4.0.x**
I see another bug - probably unlikely and maybe not even possible, but `super.stop()` is not called if
`serverChannel == null`.