CORE: Introduce a code path in "ProcessPriorityRequests" to handle headers (with no data) that have already been marked as valid
CORE: Introduce a code path in "ProcessPriorityRequests" to handle headers (with no data) that have already been marked as valid

These are essentially the equivalent of pruned blocks, and are a special case that occurs when using SPV filtering.
We also fix a very minor "bug" (not harmful in any way) with checkpoint messages

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