Function-based bibs-by-item-age enhancement

Function-based bibs-by-item-age enhancement

From Hubert Lubaczewski (a.k.a. depesz), based on investigation of
performance issues at the database layer of Evergreen, we received
an initial implementation of a replacement bibs-by-item-age mechanism.
See http://markmail.org/thread/3pe3uzokzzcqwy5b for details, but the
tl;dr version is a speedup on test data from ~13s to about 3ms.

The version offered removed some functionality, which I have attempted
to reimplement within his framework. See http://markmail.org/thread/3pe3uzokzzcqwy5b#query:+page:1+mid:bekujg5ggfydujdl+state:results
for the details of that.

Signed-off-by: Mike Rylander <mrylander@gmail.com>