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)
```
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