Randomly perform garbage collection on database tables

The following three database tables need garbage collection, clearing any rows that have been marked as expired:

```
users_remembered
users_resets
users_throttling
```

For most use cases, the best solution may be to trigger the garbage collection for a table *randomly*, e.g. during related operations, using an implementation like this:

```php
function random_float($min = null, $max = null) {
$min = isset($min) && \is_numeric($min) ? (float) $min : 0;
$max = isset($max) && \is_numeric($max) ? (float) $max : 1;

return $min + \mt_rand(0, \mt_getrandmax()) / \mt_getrandmax() * ($max - $min);
}

$probability = 0.02;

if (\random_float() < $probability) {
// perform garbage collection ...
}
```

For the `users_confirmations` table, garbage collection would be more complex. For each `user_id` and for each `email` in that table, the row with the largest `id` must be preserved so as not to break the re-sending of previous confirmations.

Start by locating the code that creates and reads the users_remembered, users_resets, users_throttling, and users_confirmations tables. Review how expired rows are represented and how confirmation rows are selected; done means expired rows are periodically removed while preserving the newest row for each user_id and email in users_confirmations.

Written by the indexing model from the issue text.