Add a bihash prefetchable bucket-level cache

Add a bihash prefetchable bucket-level cache

According to Maciek, the easiest way to leverage the csit "performance
trend" job is to actually merge the patch once verified. Manual
testing indicates that the patch improves l2 path performance. Other
use-cases are TBD. It's possible that we'll need to back out the patch
depending on what happens.

Change-Id: Ic0a0363de35ef9be953ad7709c57c3936b73fd5a
Signed-off-by: Dave Barach <dave@barachs.net>