This method takes a UserID, Key, and Value, and attempts to set $Key = $Value for $UserID.
$Key can be an SQL wildcard, thereby allowing multiple variations of a $Key to be set. $UserID
can be an array, thereby allowing multiple users' $Keys to be set to the same $Value.
++ Before any queries are run, $Key is converted to its fully qualified format (Plugin.
prepended)
++ to prevent collisions in the meta table when multiple plugins have similar key names.
If $Value == null, the matching row(s) are deleted instead of updated.