Update SCAN to handle very large cursor values. - #2458

Technically Redis may return any unsigned 64 bit integer as a scan
cursor.  This presents a problem for PHP in that PHP's integers are
signed.  Because of that if a scan cursor is > 2^63 it will overflow and
fail to work properly.

This commit updates our SCAN family of commands to deliver cursors in
their string form.

```php
public function scan(null|int|string $iterator, ...);
```

On initial entry into our SCAN family we convert either a NULL or empty
string cursor to zero, and send the initial scan command.

As Redis replies with cursors we either represent them as a long (if
they are <= ZEND_ULONG_MAX) and as a string if greater.  This should
mean the fix is minimally breaking as the following code will still
work:

```php
$it = NULL;
do {
    print_r($redis->scan($it));
} while ($it !== 0);
```

The `$it !== 0` still works because the zero cursor will be represented
as an integer.  Only absurdly large (> 2^63) values are represented as a
string.

Fixes #2454