Fix async hook assertion failure
On further testing I've noticed there was a bug in my asynchronous hook event patch (sorry for the mess): it detached
client->fetchhandle too late, so if resumed query processing attempts to do another async event or perform DNS recursion, it would trigger an assertion failure on
The first commit in this branch fixes the issue in a straightforward way. The second commit is actually irrelevant to the bug/fix, but an editorial cleanup I've noticed while looking at the unit test code to cover this bug. If it's deemed to be a distraction please ignore or defer it to a separate MR.