osd: pass and verify data+omap digest on copyfrom

Two things here:

 1- Pass the original digest from the source across the wire, if it is
    present.
 2- Calculate a new digest as we receive it, and record that.

If there is a mismatch, we currently crash; need to turn this into an
EIO, most likely.

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