Add flow control in http2

src/brpc/policy/http2_rpc_protocol.h

@@ -164,6 +164,7 @@ private:
 class H2StreamContext : public HttpContext {
 public:
     H2StreamContext();
+    ~H2StreamContext();
     void Init(H2Context* conn_ctx, int stream_id);
 
     H2StreamContext(H2Context* conn_ctx, int stream_id);
@@ -198,6 +199,7 @@ friend class H2Context;
 #endif
     bool _stream_ended;
     butil::atomic<int64_t> _remote_window_size;
+    butil::atomic<int64_t> _local_window_size;
     uint64_t _correlation_id;
     butil::IOBuf _remaining_header_fragment;
 };

src/brpc/socket.h

@@ -762,6 +762,11 @@ private:
     butil::Mutex _stream_mutex;
     std::set<StreamId> *_stream_set;
 
+    // In some protocols, certain resources may run out according to 
+    // protocol spec. For example, http2 streamId would run out after
+    // long time running and a new socket should be created. In order
+    // not to affect main socket, _agent_socket are introduced to 
+    // represent the communication socket.
     SocketUniquePtr _agent_socket;
 };