[9.20] chg: usr: named could crash on concurrent TKEY DELETE for the same key

On a server configured with tkey-gssapi-keytab (or tkey-gssapi-credential),
an authenticated peer could crash named by sending two TKEY DELETE requests
for the same dynamic key in rapid succession.  This has been fixed.

Closes #6001

Backport of MR !12041

Merge branch 'backport-6001-tsig-tkey-delete-uaf-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12042