What would you like to be added?

Part of #12913

This flag isn't needed for v3 store (bbolt), and we are going to bootstrap etcdserver from the consistent_index in v3 store (bbolt). This flag will be completely useless in 3.7.

So we need to mark --snapshot-count as deprecated in 3.6 and to be decommissioned in 3.7.

Why is this needed?

To ensure a smoother deprecation process for --snapshot-count