session: support leader-and-follower for tidb_replica_read (#14761) #15721
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Signed-off-by: qupeng qupeng@pingcap.com
What problem does this PR solve?
Cherry pick #14761 to support set tidb_replica_read = leader-and-follower in release-3.1.
What is changed and how it works?
Add a new session variable.
Related changes
Check List
Tests
Release note
Support leader-and-follower for tidb_replica_read.