DestroyUser called ListPreAuthKeys(tx) which returns ALL pre-auth keys
across all users, then deleted every one of them. This caused deleting
any single user to wipe out pre-auth keys for every other user.
Extract a ListPreAuthKeysByUser function (consistent with the existing
ListNodesByUser pattern) and use it in DestroyUser to scope key deletion
to the user being destroyed.
Add unit test (table-driven in TestDestroyUserErrors) and integration
test to prevent regression.
Fixes#3154
Co-authored-by: Kristoffer Dalby <kristoffer@dalby.cc>
7c756b8201