Overview
========

- DatagramSocket and MulticastSocket are changed to delegate to either the socket adaptor
or the old implementation. The old implemenetation is used when the static factory is
configured, jdk.net.usePlainDatagramSocketImpl is set (or set or true), or when
DatagramSocket is extended and its protected constructor is used to create a DatagramSocket
with a custom impl.

- The legacy/old implementation of DatagramSocket and MulticastSocket are copied to
DatagramSocketImplWrapper. It is non-public and overrides every public method defined
by both DatagramSocket and MulticastSocket. The implementation delegates to the
DatagramSocketImpl (either PlainDatagramSocketImpl or custom) as before.

Behavior differences
====================

- The main behavior difference between the old and "new" implementation is with connected
datagrams. Connected datagrams are emulated on macOS in the old implemenetation. The old
implementation also falls back to emulation on other platforms when the connect fails
(e.g. connect to a non-routable address). DatagramChannel discards all datagrams in the
socket read buffer when connecting the socket whereas the old DatagramSocket implementation
will buffer and filter so that datagrams are not discarded.

- If DatagramSocket or MulticastSocket is extended and the setReuseAddress, bind, or close
methods are overridden, then these methods are not invoked at construction time (the constructor
doesn't leak "this"). The old implementation calls the overridden methods at construction
time.

- The ordering that parameters are checked, the security manager check, and
the check for a closed socket may differ so the exception thrown when there are two
or more exception conditions may differ.