Fix use-after-free in concurrent dns_tsigkey_delete()

Fix use-after-free in concurrent dns_tsigkey_delete()

Two TSIG-authenticated TKEY DELETE queries for the same dynamic key,
arriving on different worker loops, could each enter
dns_tsigkey_delete() and cause over-decrementing the key refcount.

This has been fixed by making dns_tsigkey_delete() idempotent.

(cherry picked from commit 5c8dcd4419693320817e44ad144002cd7c648acc)