check max_read_stale_time when setting variables

src/share/system_variable/ob_system_variable_factory.h

@@ -1699,6 +1699,7 @@ public:
   const static int64_t MYSQL_SYS_VARS_COUNT = 97;
   const static int64_t OB_SYS_VARS_COUNT = 135;
   const static int64_t ALL_SYS_VARS_COUNT = MYSQL_SYS_VARS_COUNT + OB_SYS_VARS_COUNT;
+  const static int64_t INVALID_MAX_READ_STALE_TIME = -1;
 
   const static int16_t OB_SPECIFIC_SYS_VAR_ID_OFFSET = 10000;
   // 表示当前OB能够使用的sys var id的最大值，正常情况下，不需要申请大于OB_MAX_SYS_VAR_ID的sys var id，

src/share/system_variable/ob_system_variable_init.cpp

@@ -2882,7 +2882,7 @@ static struct VarsInit{
       ObSysVars[219].info_ = "max stale time(us) for weak read query " ;
       ObSysVars[219].name_ = "ob_max_read_stale_time" ;
       ObSysVars[219].data_type_ = ObIntType ;
-      ObSysVars[219].value_ = "5000000" ;
+      ObSysVars[219].value_ = "-1" ;
       ObSysVars[219].flags_ = ObSysVarFlag::GLOBAL_SCOPE | ObSysVarFlag::SESSION_SCOPE | ObSysVarFlag::NEED_SERIALIZE ;
       ObSysVars[219].on_check_and_convert_func_ = "ObSysVarOnCheckFuncs::check_and_convert_timeout_too_large" ;
       ObSysVars[219].id_ = SYS_VAR_OB_MAX_READ_STALE_TIME ;