Avoid race condition that could result in NPE

[hannahhoward]

Goals

When the same custom data store is used on two seperate requests on the requestor, an NPE can occur in a specific case:

1. When the responder receives ExtensionDeDupByKey, it sets up a specific link tracking de-deduplication list for the first request
2. When the second request is received, it reuses the same link tracking de-duplication list.
3. If the first request finishes before the second request sends a single link, the specific link tracking list will get cleaned up because the check is simply whether the list has been cleared of all links, which given a finished request and a request that has not started traversing, will be the case.
4. When the second request attempts to use the link tracking list, it will get an NPE

Implementation

Change the check for cleanup of the link tracking list to be an iteration of all deduplicated requests to confirm that NO requests reference this particular de-duplication list, rather than a check of whether the list itself is empty.

[arajasek]

Makes sense to me!