VariableMgr flag could be simplified #1220
Description
I think the following three flags could simplify to one flag GLOBAL_MODIFY_ONLY, which means the variable could only modify in global level, if a variable no any flag, which could modify in global level and session level both.
// variable have this flag means that every session have a copy of this variable,
// and can modify its own variable.
public static final int SESSION = 1;
// Variables with this flag have only one instance in one process.
public static final int GLOBAL = 2;
// Variables with this flag only exist in each session.
public static final int SESSION_ONLY = 4;