When etcdserver receives a LeaseRenew request, it may be still in
progress of processing the LeaseGrantRequest on exact the same
leaseID. Accordingly it may return a TTL=0 to client due to the
leaseID not found error. So the leader should wait for the appliedID
to be available before processing client requests.
I think strong (not-equal) relationship was too restrictive when expressed with 1s granularity.
```
logger.go:130: 2022-04-03T22:15:15.242+0200 WARN m1 leader failed to send out heartbeat on time; took too long, leader is overloaded likely from slow disk {"member": "m1", "to": "cb785755eb80ac1", "heartbeat-interval": "10ms", "expected-duration": "20ms", "exceeded-duration": "24.666613ms"}
logger.go:130: 2022-04-03T22:15:15.262+0200 INFO m-1 published local member to cluster through raft {"member": "m-1", "local-member-id": "e2dd9f523aa7be87", "local-member-attributes": "{Name:m-1 ClientURLs:[unix://127.0.0.1:2196386040]}", "cluster-id": "b4b8e7e41c23c8b5", "publish-timeout": "5.2s"}
v3_lease_test.go:415: Expected lease ttl (4m58s) to be greather than (4m58s)
```
Current checkpointing mechanism is buggy. New checkpoints for any lease
are scheduled only until the first leader change. Added fix for that
and a test that will check it.
