[2.1] fixed tons of missing/wrong use statements

On the following errors I need help to resolve
- `Zend\Db\Metadata\Source\AbstractSource` uses the not existing class `Object\ConstraintKeyObject`
- `Zend\XmlRpc\Server\Exception\Stdin::____construct()` uses `new ServerException` which is defined as `use Zend\XmlRpc\Server\Exception as ServerException`
- `Zend\XmlRpc\AbstractValue::_phpVarToNativeXmlRpc` checks against the not existing class `Zend\Math\BigInteger`