osd: change omap data encoding in object_copy_data_t

Pass the omap data in the struct as an opaque (encoded) bufferlist.  This
avoids a decode into an STL map, simplifying the crc calculation.  The
win isn't huge (yet) because we still turn it into a map to pass it down
to the ObjectStore method, but that too will change in time.

Signed-off-by: Sage Weil <sage@redhat.com>
osd: scrub: wait for digest updates to apply before next scrub chunk

Wait for any digest updates to apply before we scrub the next chunk.  This
bounds the number of repops we initiate by the size of the scrub chunk, and
it generally nicer to the cluster.

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