Remove the nested read loops in FastCGIConn, hide complexity inside StreamBuffer

stream_buffer.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "buffer.h"
+
+class StreamBuffer : public Buffer {
+  public:
+	StreamBuffer(int sock, size_t size);
+
+	[[nodiscard]] bool BufferAtLeast(size_t len);
+	[[nodiscard]] const char *Read(size_t len) override;
+	template<class T> [[nodiscard]] const T *ReadObj();
+
+  private:
+  	int sock_;
+};
+
+template<class T> const T *StreamBuffer::ReadObj() {
+	return reinterpret_cast<const T*>(Read(sizeof(T)));
+}