…lied on v2store

ClusterVersionSet, ClusterMemberAttrSet, DowngradeInfoSet functions are
writing both to V2store and backend. Prior this CL there were
in a branch not executed if shouldApplyV3 was false,
e.g. during restore when Backend is up-to-date (has high
consistency-index) while v2store requires replay from WAL log.

The most serious consequence of this bug was that v2store after restore
could have different index (revision) than the same exact store before restore,
so potentially different content between replicas.

Also this change is supressing double-applying of Membership
(ClusterConfig) changes on Backend (store v3) - that lackilly are not
part of MVCC/KeyValue store, so they didn't caused Revisions to be
bumped.

Inspired by jingyih@ comment:
etcd-io#12820 (comment)