Fix performace issues for over 30k users

When Safety Net is activated on a site with a large number of users, it takes a while to run its auto-deletion, sometimes resulting in timeouts or 504s.

30k users on Pressable, it finishes after about 4.5 minutes, just under what I'm assuming is the 5 minute PHP limit
50k users, it 504s

You can test this by spinning up a test site on Pressable, and using wp user generate --count=10000 a few times to generate fake users with meta.

If you want to test more than once on the same site, you'll also need to delete the safety_net_data_deleted option that gets set after the deletion function completes.

This came up here: #46

I think the solution to this is going to be to remove the foreach loop here:

safety-net/includes/delete.php

Lines 42 to 53 in 0c42f1c

    
           foreach ( $users as $user ) { 
        
           	// Skip administrators. 
        
           	if ( in_array( $user->ID, $admins, true ) ) { 
        
           		continue; 
        
           	} 
        
           	// Get all of their meta and delete it. 
        
           	delete_all_users_meta( $user->ID ); 
        
           	// Delete the user. 
        
           	$wpdb->delete( $wpdb->users, array( 'ID' => $user->ID ) ); 
        
           }

And replace it with 2 DB calls to delete the usermeta and users that are NOT IN an array of admin user IDs.

Fixed here: #57

Tested at 60k users in 9 seconds.

	foreach ( $users as $user ) {
	// Skip administrators.
	if ( in_array( $user->ID, $admins, true ) ) {
	continue;
	}

	// Get all of their meta and delete it.
	delete_all_users_meta( $user->ID );

	// Delete the user.
	$wpdb->delete( $wpdb->users, array( 'ID' => $user->ID ) );
	}