osd: Add digest of omap for deep-scrub

Add ScrubMap encode/decode v4 message with omap digest
Compute digest of header and key/value.  Use bufferlist
to reflect structure and compute as we go, clearing
bufferlist to reduce memory usage.

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>