buffer: instrument utilization of cached crcs

This is similar to the alloc tracking, but I've added a method to let
you enable it easily (in this case, from the unit test).

Signed-off-by: Sage Weil <sage@inktank.com>
buffer: add crc caching performance test

On my old box:
 - matching cached values is a big win (free), obviously
 - the adjustment is the same speed as redoing the calculation.  this
   is probably because the data is already in L1/L2 cache; we still
   save memory bandwidth.

Signed-off-by: Sage Weil <sage@inktank.com>
buffer: explicitly init zbuf to zeros

This might not be necessary?

Signed-off-by: Sage Weil <sage@inktank.com>