librados: add OPERATION_ORDER_READS_WRITES flag

This maps to the internal RWORDERED option, which indicates that a read
should be ordered the same way a write is.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph_test_rados: order racing read wrt to the COPY_FROM

The prevents:

 - send COPY_FROM, blocks on recovering object
 - send stat, procedes to return old object version
 - COPY_FROM succeeds with new version
   -> fail

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