vstreamer: remove table locks for snapshots

[derekperkins]

Per @sougou in #8909, the snapshot connection should already be sufficient without introducing extra lock contention.

Related Issue(s)

May fix #8909

Checklist

• I would like this to make it into the v12 release

[sougou]

I do think we need the lock. It's because the gtid we compute after starting a transaction can still change, and does not represent the gtid as of when the transaction was started.

[shlomi-noach]

Agreed, the lock is required for GTID. However, as of MySQL 8.0.17, the table mysql.gtid_executed is always in sync with actual gtid_executed (before that version it would only update periodically and could represent older GTID set). Which means as of 8.0.17 we can potentially remove the lock and instead query for the GTID via SELECT * FROM mysql.gtid_executed inside the consistent snapshot.

Then again, this work on paralellising vstreamer copy: #8934, yet again required the lock and cannot work without it.

So I'd say let's keep the lock for now.