httpgateway: set new max payload length config value

config.h

@@ -14,6 +14,7 @@ class Config
 
 	std::string optional(const std::string &key, std::string defaultvalue = "");
 	int optional(const std::string &key, int defaulvalue);
+	uint64_t optional(const std::string &key, uint64_t defaultvalue);
 
   public:
 	Config(const std::map<std::string, std::string> &map);
@@ -51,6 +52,9 @@ class Config
 	int session_max_lifetime;
 	int max_pagename_length;
 	int threadscount;
+
+	uint64_t max_payload_length;
+
 	Permissions anon_permissions;
 
 	std::string getConfig(const std::string &key) const