Allow to purge bunch of transactions while slave is running

[Sveta Smirnova]

With multi-source replication, when you want to add new master to existing slave you may need to purge old GTIDs from backup. If you have small number of transactions you can easily skip them by inserting empty transactions while second replication channel is running. But if you have to inject really large number of transactions (say, 1M) it is impractical even with help of scripting/utilities.

Please implement one of:

- allow committing a [empty] gtid set instead of a single gtid, though this would require changes in how binlogs are parsed
- allow appending gtid sets to gtid_purged, even if just for UUIDs that aren't currently represented in the extant sets.
- allow changing purged while transactions are open, as long as no purge is running, as those transactions would only update gtid_executed and the binary log, and don't actually change gtid_purged.

[Sveta Smirnova (svetasmirnova)]

Probably https://bugs.mysql.com/bug.php?id=84994 maybe implemented in a way which allow to solve this issue with multi-channel replication setup.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2509