[9.20] fix: usr: Fix rare assertion failure when shutting down incoming transfer

A very rare assertion failure can be triggered when the incoming transfer is either forcefully shut down or it is finished during printing the details about the statistics channel.  This has been fixed.

Closes #4860

Backport of MR !9336

Merge branch 'backport-4860-destroy-xfrin-timers-on-the-loop-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9377