libceph: support for balanced and localized reads

OSD-side issues with reads from replica have been resolved in
Octopus.  Reading from replica should be safe wrt. unstable or
uncommitted state now, so add support for balanced and localized
reads.

There are two cases when a read from replica can't be served:

- OSD may silently drop the request, expecting the client to
  notice that the acting set has changed and resend via the usual
  means (handled with t->used_replica)

- OSD may return EAGAIN, expecting the client to resend to the
  primary, ignoring replica read flags (see handle_reply())

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>