DTR priority boosting

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This merge request is automatically copied from !476 (merged) orginally made by @dcameron
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This change was requested by Andrej. He found that if there was a low priority transfer stuck in the queue, it would block a higher priority transfer which requests the same source file. With this change, if a new transfer with higher priority comes along, the lower priority transfer in the queue will have its priority boosted to that of the higher priority transfer.