fix(queue): read running under mutex in loop to avoid data race

[Noethix55555]

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Fixes #907

Problem

Queue.loop() read the running field without holding q.mut, while Stop() and Run() write it under q.mut. This is a data race, reported by go test -race:

WARNING: DATA RACE
Write at ... by goroutine N: queue.(*Queue).Stop()  queue/queue.go:156
Previous read at ... by goroutine M: queue.(*Queue).loop()  queue/queue.go:168

Fix

Snapshot running under q.mut at the top of each loop iteration and use the local copy for the termination check.

Test

Added TestQueueStopRace: queues several thousand requests, then calls Stop() from another goroutine while Run() is processing. Run with -race it fails on master and passes with this change. The full go test -race ./queue/... suite passes.
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