Convert::memstring2bytes should return integer value

[wernerkrauss]

bytes are by nature an integer

fixes #8572

[robbieaverill]

This is potentially a semver violation. If people were to introspect the return type of this method they'd see it change from float to int, but seeing as we always round the float return value of round(), it'd be an int anyway. LGTM 👍

[unclecheese]

FYI This broke the silverstripe/assets test, which is expecting a float. https://github.com/silverstripe/silverstripe-assets/blob/1/tests/php/FileTest.php#L701

[robbieaverill]

Ha! Damn. Do you think we should revert @unclecheese, or update that test?