[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_server/cdc_rsync_server.h

@@ -32,6 +32,7 @@ namespace cdc_ft {
 
 class MessagePump;
 class ServerSocket;
+class SocketFinalizer;
 
 class CdcRsyncServer {
  public:
@@ -90,6 +91,8 @@ class CdcRsyncServer {
   // Used to toggle decompression.
   void Thread_OnPackageReceived(PacketType type);
 
+  // The order determines the correct destruction order, so keep it!
+  std::unique_ptr<SocketFinalizer> socket_finalizer_;
   std::unique_ptr<ServerSocket> socket_;
   std::unique_ptr<MessagePump> message_pump_;