PlannedReparent: call SetReadOnly in PromoteReplica only after master tablet is written to topo
PlannedReparent: call SetReadOnly in PromoteReplica only after master tablet is written to topo

Signed-off-by: deepthi <3f83f38ad2e2ddcc47e21eb12a05ce66eb4cf525@planetscale.com>

mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess