GetGlobalSystemVar gets a global system variable.

GetScopeNoneSystemVar checks the validation of `key`, and return the default value if its scope is `ScopeNone`.

GetSessionOnlySysVars get the default value defined in code for session only variable.

GetSessionSystemVar gets a system variable.

GoTimeToTS converts a Go time to uint64 timestamp.

SetSessionSystemVar sets system variable and updates SessionVars states.