Reason to store CI and term in backend was to make db fully independent
snapshot, it was never meant to interfere with apply logic. Skip of CI
was introduced for v2->v3 migration where we wanted to prevent it from
decreasing when replaying wal in
https://github.com/etcd-io/etcd/pull/5391. By mistake it was added to
apply flow during refactor in
https://github.com/etcd-io/etcd/pull/12855#commitcomment-70713670.
Consistency index and term should only be negotiated and used by raft to make
decisions. Their values should only driven by raft state machine and
backend should only be responsible for storing them.
Prior to this CL, `ETCDCTL_API=2 etcdctl backup --with-v3` was readacting WAL log
(by removal of some entries), but was NOT updating consistent_index in the backend.
Also the WAL editing logic was buggy, as it didn't took in consideration the fact
that when TERM changes, there can be entries with duplicated indexes in
the log. So its NOT sufficient to subtract number of removed entries to
get accurate log indexes.
The PR replaces removing and shifting of WAL entries with replacing them with an no-op entries.
Thanks to this consistent-index references are staying up to date.
The PR also:
- updates 'verification' logic to check whether consistent_index does not lag befor last snapshot
- env-gated execution of verification framework in `etcdctl backup`.
Tested with:
```
(./build.sh && cd tests && EXPECT_DEBUG=TRUE 'env' 'go' 'test' '-timeout=300m' 'go.etcd.io/etcd/tests/v3/e2e' -run=TestCtlV2Backup --count=1000 2>&1 | tee TestCtlV2BackupV3.log)
```
