fixed possible datarace for downloader queue

[jeongkyun-oh]

Proposed changes

• This PR is derived from eth/downloader: fix possible data race by inconsistent field protection ethereum/go-ethereum#20690 to prevent potential data race for downloader queue.
• The queue lock is acquired after calling the function, which means the lock is acquired after pendPool read.

Types of changes

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• Bugfix
• New feature or enhancement
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[aidan-kwon]

@jeongkyun-oh I cannot find any relation with ethereum/go-ethereum#21201. Indeed, the old code looks better for me.

[jeongkyun-oh]

@aidan-kwon Sorry, it is related to 20690. The lock is acquired in each cancel function, which may cause data-race if other threads access the pending pool. The point is that the lock is acquired after reading pendPool which may be written by other goroutines.