rbd: implement sync object read with new code

Reimplement the synchronous read operation used for reading a
version 1 header using the new request tracking code.  Name the
resulting function rbd_obj_read_sync() to better reflect that
it's a full object operation, not an object request.  To do this,
implement a new OBJ_REQUEST_PAGES object request type.

This implements a new mechanism to allow the caller to wait for
completion for an rbd_obj_request by calling rbd_obj_request_wait().

This partially resolves:
    http://tracker.newdream.net/issues/3755



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>