Support pause and resume of MySQL cluster clustering.

[d-kuro]

What

Support pause and resume of MySQL cluster clustering.

There is a user need to align the gtid_executed of all replicas manually
by doing replication from each Pod for consistency checks between Pods in the MySQL Cluster.
In this process, it is necessary to stop the replica's sql thread and manually advance the gtid,
but there is no function to stop clustering in the current MOCO and MOCO starts replication automatically.
Therefore, we are implementing a function to temporarily stop MOCO's clustering monitoring.

How

Proposals will be prepared.

Checklist

• Finish implementation of the issue
• Test all functions
• Have enough logs to trace activities
• Notify developers of necessary actions

[d-kuro]

• Stop reconciliation and clustering #578

I Implemented in the above pull request.
This change is included in v0.20.0 of moco.

https://github.com/cybozu-go/moco/releases/tag/v0.20.0