P2PServer: ask peers about unknown blocks only once

src/p2p_server.h

@@ -174,6 +174,9 @@ private:
 	uv_async_t m_broadcastAsync;
 	std::vector<Broadcast*> m_broadcastQueue;
 
+	uv_mutex_t m_missingBlockRequestsLock;
+	std::set<std::pair<uint64_t, uint64_t>> m_missingBlockRequests;
+
 	static void on_broadcast(uv_async_t* handle) { reinterpret_cast<P2PServer*>(handle->data)->on_broadcast(); }
 	void on_broadcast();
 };