Prepopulate was setting entry.Expiry.HeapIndex to 0. Previously this would result in a call to heap.Fix(0)
which wasn't correct, but was also not really a problem because at worse it would re-notify.
With the recent change to extract cachettl it was changed to call Update(idx), which would have updated
the wrong entry.
A previous commit removed the setting of entry.Expiry so that the HeapIndex would be reported
as -1, and this commit adds a test and handles the -1 heap index.
