Bootstrap etcdserver from v3store (consistent_index)

[ahrtr]

What would you like to be added?

We have already deprecated v2store, and v3store is the solely source of truth for all data, including membership data in 3.6.0. But etcdserver still bootstrap from v2store, the latest v2 snapshot file. It will replay all WAL records starting from the latest snapshot index.

It doesn't make sense. We should bootstrap etcdserver from v3store, and replay WAL records start from the consistent_index.

It's the last blocker to completely get rid of the v2store.

Previous draft #17022

Why is this needed?

to completely get rid of v2store

[ahrtr]

Regarding the test, I think we can add a verification (see an example #19730) to ensure that etcdserver never applies entries <= consistent_index.

[hwdef]

/cc

[ahrtr]

/assign @fuweid