alter table _pico_replicaset add column promotion_vclock

Summary

• chore: update TODO comment

• feat: persist promotion_vclock when switching replicaset master

• fix: make proc_get_vclock work with ConnectionPool::call

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• Close #857 (closed), a part of #856 (closed)
• Cherry-pick to: none
• Docs follow-up: updated _pico_replicaset schema