[cdc_rsync] Add support for ServerSocket on Windows (#48)

Makes ServerSocket multi-platform, mainly by working around some small
API differences. The code is largely the same, there should be no
differences on Linux.

Also moves WSAStartup() and WSACleanup() up to the Socket level as
static methods because it's used by both ClientSocket and ServerSocket,
and because it doesn't make sense to do that in the socket class as
that would prevent one from using several sockets.

cdc_rsync/cdc_rsync_client.h

@@ -123,6 +123,7 @@ class CdcRsyncClient {
   WinProcessFactory process_factory_;
   RemoteUtil remote_util_;
   PortManager port_manager_;
+  std::unique_ptr<SocketFinalizer> socket_finalizer_;
   ClientSocket socket_;
   MessagePump message_pump_{&socket_, MessagePump::PacketReceivedDelegate()};
   ConsoleProgressPrinter printer_;