librados, osd: add flags to COPY_FROM

librados, osd: add flags to COPY_FROM

If we initiate a COPY_FROM as part of a FLUSH operation, we will need to
set a flag so that the read-side of the copy and join the existing
in-progress operation without taknig additional locks.

Similarly, we need to pass flags from the client indicating whether we
should ignore overlay or cache logic while performing the copy.  These are
used by the promote and flush logic.

Note that none of these flags are exposed through librados (at least not
at this time).

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