ShaBloom prefilter to detect nonexistant objects

ShaBloom prefilter to detect nonexistant objects

This inserts a bloom prefilter ahead of midx for efficient checking of
objects most of which do not exist.  As long as you have enough RAM for
the bloom filter to stay in memory, this saves a lot of time compared to
midx files.  Bloom filter is between 1/5th and 1/20th the size of midx
given the parameters I'm using so far.

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