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.

To extend lease checkpointing mechanism to cases when the whole etcd
cluster is restarted.

…lease-checkpoint-persist to persist lease remainingTTL

To avoid inconsistant behavior during cluster upgrade we are feature
gating persistance behind cluster version. This should ensure that
all cluster members are upgraded to v3.6 before changing behavior.

To allow backporting this fix to v3.5 we are also introducing flag
--experimental-enable-lease-checkpoint-persist that will allow for
smooth upgrade in v3.5 clusters with this feature enabled.