[improve][broker] Improve `retention` endpoint to pure async.

[mattisonchao]

Motivation

Since Pulsar is a fully asynchronous project, we need to avoid using thread-blocking methods. Also, we can see https://github.com/apache/pulsar/issues/14365

Modifications

• Improve backlogQuota relative endpoint to pure async.
• Remove unused method validatePolicies.

Verifying this change

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• Extended integration test for recovery after broker failure

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• Anything that affects deployment: (yes / no / don't know)

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