SERVER-78409 modified removing (erasing) a cancelled client from _outstandingQueries by query instead of an iterator (which can be invalid). New behavior is to traverse the linked list to find query and remove. Since the list should be small, there should be no performance impact, however we need to add a test to measure the performance. It may be especially important if we have to cancel many client connections at the same time, thus performing a linear lookup in the list for every client entry. If it turns out that there is a performance issue, we should consider replacing the list with a map.