It was found that if a fetch was started for a specific `Vec<RefsAt>`
and a following general, i.e. empty, `Vec<RefsAt>` was queued, the
subscriber for the general one would be removed when the specific
fetch completed.
This behaviour is undesirable, since the general fetch could
potentially fetch more data.
To prevent this from happening, the `Vec<RefsAt>` is added to the
`FetchKey`, so that `subscribers` are matched with the set of
references they want to hear results for.
A test was added for the above scenario.
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