find-links-from-to-three

[Eric Dean Tribble <tribble>]

I just realized while talking to marcs that we can now support
find-links-from-to-three-home reasonably efficiently.  The current
link operation corresponds to find-links-from-three-home or
find-links-to-three-home.  The combination of two operations (one on
the from set, one on the to-set) does the full flftt operations.  All
that's required is a hash set to find the intersection of the two
results.

dean