[MERGE][#354036] Server-side (sink-side) fix for missing inventory parents

[Andrew Bennetts]

Robert Collins wrote:
> Robert Collins has voted comment.
> Status is now: Semi-approved
> Comment:
> I may be misreading it but this seems like it will reprocess a bunch of  
> data we've already processed.
>
> Wouldn't it be much better to accumulate the references as data comes  
> through?

Hmm.  Well, better from a performance point of view I suppose!  I had erred
on the side of simple and clearly robust, but I guess we do want to be
careful to keep this code fairly performant.

So here's an updated patch that accumulates the references as the data comes
through.  It required a bit more care to make this version work correctly
with resumed write groups, but the extra tests should be enough to verify
that this does indeed work.  Alarmingly the scan_unvalidated_index method
was never being called (except by the test suite) without this patch!

-Andrew.