extend the semantics of tidb_mem_quota_query

[XuHuaiyu]

Enhancement

tidb_mem_quota_query indicates the memory usage threshold of a query. But it's not applicable when SQL is in a transaction. During the execution of the transaction, different SQL will access the same memBuffer, and the memory of memBuffer will be released after commit or rollback. The memory usage of memBuffer should be accumulated to the memory tracker of a session but not a query.
We need to extend the semantics of tidb_mem_quota_query. The new definition is the memory usage threshold of a session. Actually, the new definition is more intuitive. For a query that is not in a transaction, there is no difference when the memory usage touches the threshold.