Rethink clientv3 stream API error handling

[gyuho]

Current behavior

• Lease KeepAlive/KeepAliveOnce does not return corresponding context error, even if context is canceled or timed out (proposal clientv3: handle context error at beginning in KeepAlive #9036)
  • It only closes the response channel
• Lease KeepAlive/KeepAliveOnce should just return response channel, where we can post errors (just like Watch interface--proposal clientv3: don't halt lease client if there is a lease error #7866)

https://github.com/coreos/etcd/blob/b0a7623be82c332942994ac2a09c70d930ebbb77/clientv3/lease.go#L132-L139

https://github.com/coreos/etcd/blob/b0a7623be82c332942994ac2a09c70d930ebbb77/clientv3/watch.go#L49

[stale]

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