OSD: use OpRequestRef& for a few require_* functions

Signed-off-by: Greg Farnum <greg@inktank.com>
OSD: introduce require_self_aliveness(OpRequestRef&,epoch_t) function

Take the self-aliveness checks out of require_same_or_newer_map() and use
the new function for that and for require_up_osd_peer().

Signed-off-by: Greg Farnum <greg@inktank.com>
OSD: add require_same_peer_inst(OpRequestRef&,OSDMap&) helper

Signed-off-by: Greg Farnum <greg@inktank.com>