Reduce etcd dependency on store v2

[ptabor]

StoreV2 is significant code-complexity and performance 'debt' of etcd.
There was plan to decommission the code as part of etcd-3.6 release. But to make it happen we need to
get rid of all read-only dependencies in etcd-3.5 such that the migration 3.5 -> 3.6 and potential rollback 3.6 -> 3.5 will be safe.

Performance benefits:

• no need to apply_v2
• raft snapshots will be very lightweight - a few bytes in WAL log: so we will be able to snapshot every 1000 entries.
  No additional files. Recovery / boot-time should be decrease.

What needs to be done:

• Snapshot's ConfState need to be written to WAL: no-store_v2: Store ConfState as part of WAL log snapshot #12735
• Optional: Store ConfState in bolt (for consistency - to make bolt self-sufficient consistent snapshot)
• Replace publish with publishV3. Sound trivial, but I had failed in the first attempt.
• Membership information should be bootstraped from Backend(+WAL), not from v2store:

  etcd/server/etcdserver/api/membership/cluster.go

  Line 251 in 1b300cd

  c.members, c.removed = membersFromStore(c.lg, c.v2store)

• Optional: Introduce experimental flag to stop writing v2store at all.

[hexfusion]

love to see this thanks for driving @ptabor let me know if I can be of any assistance in the effort.

[ptabor]

@hexfusion: To make it happened I would appreciate a lot of help with reviews, e.g.
#12804

There seems to be a lot of 'bugs' around membership management, that I need to sort out.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 21 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 21 days if no further activity occurs. Thank you for your contributions.