lease: Client should stop sending keepAlive requests after recv channel closed

[zhijun42]

If a lease client sets this withRequireLeader and closes its receive channel due to no leader, it should also stop sending more keepAlive requests to the server (it wouldn't have a channel to receive response anyway).

I extended the existing test function TestLeaseWithRequireLeader to verify this update.

Also fixed the bug of newIdx. Surprisingly this wasn't caught for the past 9-ish years.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @zhijun42. Thanks for your PR.

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