[cdc_rsync] Fix issue in UnzstdStream (#59)

Fixes an issue in UnzstdStream where the Read() method always tries to
read new input data if no input data is available, instead of first
trying to uncompress. Since zstd maintains internal buffers,
uncompression might succeed even without reading more input, so this
is faster. This bug can lead to pipeline stalls in cdc_rsync.

cdc_rsync/base/fake_socket.cc

@@ -39,7 +39,8 @@ absl::Status FakeSocket::Receive(void* buffer, size_t size,
   *bytes_received = 0;
   std::unique_lock<std::mutex> lock(data_mutex_);
   data_cv_.wait(lock, [this, size, allow_partial_read]() {
-    return allow_partial_read || data_.size() >= size || shutdown_;
+    size_t min_size = allow_partial_read ? 1 : size;
+    return data_.size() >= min_size || shutdown_;
   });
   if (shutdown_) {
     return absl::UnavailableError("Pipe is shut down");