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 isc_nmhandle_attach.

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.