Combine and speed up idx->midx and bupindex merge

Combine and speed up idx->midx and bupindex merge

These two processes used almost identical algorithms, but were
implemented separately.  The main difference was one was ascending and
the other was descending.

This patch reverses the cmp on index.Entry so that both can share an
algorithm.

It also cuts some overhead in the algorithm by using it.next() instead of
the next() wrapper, yielding a ~6% speedup on midx generation and index merging.

Signed-off-by: Brandon Low <lostlogic@lostlogicx.com>