fix: make set/read timeout and write/put buffer atomic

[youzhixiaomutou]

When I ran stress testing in prod environment, I found the buffer which is used by current request will be written by the previous request which has timeout. I found that setting tw.timeout = true and put buffer to pool is guarded by sync.mutex: https://github.com/gin-contrib/timeout/blob/c9f1fc9648482a03867dae552b1ef630600bb03c/timeout.go#L82-L86 But reading timeout is unsafed, so that it may cause a data race? I think set/read timeout and write/put buffer should be atomic, otherwise it will cause writing buffer which has already been put back to pool. https://github.com/gin-contrib/timeout/blob/c9f1fc9648482a03867dae552b1ef630600bb03c/writer.go#L29-L61

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All modified and coverable lines are covered by tests :white_check_mark:

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[appleboy]

@youzhixiaomutou Thanks.