objectcache: Optimise array_map in MemcachedBagOStuff::makeKey()

objectcache: Optimise array_map in MemcachedBagOStuff::makeKey()

This can get called a lot on an average page load, optimise
a bit by using a referenced foreach iteration instead.

Using a simplified test case, I found this saves about 70% on PHP 7.2.
For 100 iterations, from ~1.5ms to ~0.4 ms.

```lang=php
$args = [ 'FooBar', 'thisthat', 4, 'foo', 12 ];
$left = 100;
foreach ($args as &$arg) {
    $arg = strtr( $arg, ' ', '_' );
    if ( strlen( $arg ) < $left || true ) {
        $arg = '#' . $arg;
    }
    $left--;
}
```

Change-Id: Ie779c4661306a6d3dc08c08671f1a36682ca1afb