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