filters: ensure that filter unsubscription requests are not blocked

[yoomee1313]

Proposed changes

• This PR is from A potential dead lock of client which result in block syncing halt ethereum/go-ethereum#22131
• In the original PR, timeout field for unit test is added at PublicFilterAPI. Rather than remove it, it is better to keep it for the flexible timeout settings in future tests.

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Further comments

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[blukat29]

LGTM except minors.