[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/base/socket.h

@@ -26,6 +26,14 @@ class Socket {
   Socket() = default;
   virtual ~Socket() = default;
 
+  // Calls WSAStartup() on Windows, no-op on Linux.
+  // Must be called before using sockets.
+  static absl::Status Initialize();
+
+  // Calls WSACleanup() on Windows, no-op on Linux.
+  // Must be called after using sockets.
+  static absl::Status Shutdown();
+
   // Send data to the socket.
   virtual absl::Status Send(const void* buffer, size_t size) = 0;
 
@@ -40,6 +48,12 @@ class Socket {
                                size_t* bytes_received) = 0;
 };
 
+// Convenience class that calls Shutdown() on destruction. Logs on errors.
+class SocketFinalizer {
+ public:
+  ~SocketFinalizer();
+};
+
 }  // namespace cdc_ft
 
 #endif  // CDC_RSYNC_BASE_SOCKET_H_