kv: PROSCRIBED lease cannot be replaced immediately after restart

[nvanbenschoten]

Now that leaseholders can serve reads in the future of their clock time but below their lease's expiration time, we need to be more careful about when we replace leases after a restart. We currently prevent a leaseholder from re-using its lease after a restart (when it would have lost its timestamp cache), but we allow the leaseholder to immediately re-acquire a lease (with a new start time). This is no longer sufficient to ensure that the new lease doesn't serve writes that violate reads served under the old lease. Instead, we now need to prevent a new lease from beginning until after the old lease expires.

Jira issue: CRDB-3218

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