We don't need to return a closure or have a separate hash
for sorting threads by relevance. Instead, we can stuff
the relevance {pct} into the SearchMsg object itself and
use that.
Note: upon reviewing this code, the sort-by-relevance seems
bogus as it only considers the relevance of the topmost message.
Instead, it would make more sense to the user to sort by the
highest relevance of all messages in that particular thread.