Remove Zend\Stdlib\DateTime in favor of native DateTime usage

Zend\Stdlib\DateTime was introduced for no real reason; at least not without looking more deeply into how the native DateTime object works. The main reason was to be able to parse fractional seconds in ISO8601 dates, which was not possible with DateTime::createFromFormat(), but is actually possible with the DateTime constructor itself.

This PR removes that unnecessary class again and puts the DateTime constructor in place where needed.