ShaBloom: Add k=4 support for large repositories

ShaBloom: Add k=4 support for large repositories

Comments pretty much tell the story, as 3TiB is really not large enough
for a backup system to support, this adds k=4 support to ShaBloom which
lets it hold 100s of TiB without too many negative tradeoffs.  Still
better to use k=5 for smaller repositories, so it switches when the
repository exceeds 3TiB.

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