When all servers in a raft cluster are offline, the client's watch requests will loop indefinitely.

[funky-eyes]

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Ⅰ. Issue Description

当server在raft模式下全部宕机时，客户端将会无限循环尝试watch，但是最终都失败，因为内层的dopost方式将io异常catch了，导致无法sleep 1s
When the servers are all down in draft mode, the client will try to watch in an infinite loop, but eventually fail because the inner dopost method catches the io exceptions, making it impossible to sleep for 1s.

Ⅱ. Describe what happened

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Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

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Ⅴ. Anything else we need to know?

Ⅵ. Environment:

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