osd: COPY_GET operation

Add new rados operation to copy all user-visible content for an object
in a simple, safe way.  Use a new object_copy_cursor_t to keep track of
our position.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: do not requeue if not primary

This saves us a bit of work, since we will discard the op anyway if
we aren't primary (or even if we become primary again before we get to
it).

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