[Enhancement]: Use serializable read instead of linearizable read for etcd requests

[LoveEachDay]

Is there an existing issue for this?

• [X] I have searched the existing issues

What would you like to be added?

Milvus have several periodically read requests for etcd such as:

1. rootcoord import manager will periodically check import tasks
2. config will also periodically read milvus config from etcd under

And by default etcd client's Get request is linearizable, we'd provide a WithSerializable() option.

Why is this needed?

Linearizable read is relatively heavy, we can refactor our code to decrease the impact for etcd.

Anything else?

No response

[yanliang567]

/assign @xiaofan-luan

[stale[bot]]

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