When the router checker is enabled, routes are regularly marked as out
of date w.r.t. discovery. This can cause upper level messages to be
delayed while the router undergoes discovery. We can avoid delaying
messages by relying on the router checker to initiate discovery of
routers. If we happen to send a message to a router before it has
been discovered then the worst case scenario is that the route is
actually down or we end up utilizing a subset of a multi-rail router's
interfaces. Both situations can be remedied by utilizing the
check_routers_before_use parameter.

Change the logic in lnet_handle_find_routed_path() so that we only
initiate discovery if the alive_router_check_interval is <= 0 (i.e.
router checker pings are disabled).