AliSQL: [Feature] Issue#16 OMIT MAINTAINING OWNED_GTID SET IF GTID IS AUTO-GENERATED

Description:

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Before this patch, the life cycle for a GTID like the following:

Generate a GTID => Add the GTID into owned_gtid => Flush the GTID into binlog

=> Remove the GTID from owned_gtid. Owned_gtid can be treated as an intermediate

set to hold all of the GTIDs of current thread.

After this patch, the life cycle for such a GTID can be like this:

Generate a GTID => Add the GTID into binlog. Of course, if anything

wrong happens, we have to remove the GTID from the binlog. Before this

patch, we can remove the GTID from owned_gtid. Obviously, we shortened

the life cycle of GTID.

In order to support this feature, we introduced a new system variable

"rds_gtid_precommit". If it's on, we will change the life cycle of GTID.