qmanager/dfu: satisfiable but unreservable job support

problem: jobs that are satisfiable but unreservable, in queues that use
reservation, would fail with either a match failure or an EINVAL return
from resource.  This seems to only happen when `schedule` returns -1,
an errno of ENOENT and the `at` value equal to the end of the graph.

Solution: Rather than making that EINVAL, it now passes out EBUSY, which
causes the new code for blocked constraints to pull the job out of
pending and place it in blocked.  To allow the job to proceed, we move
blocked jobs back into the pending queue whenever a job is removed from
the queue due to completion or cancellation. We probably consider them
more than necessary, but this seems to solve the issue and still
preserves the performance improvement.

resource/traversers/dfu.cpp

@@ -310,7 +310,14 @@ int dfu_traverser_t::run (Jobspec::Jobspec &jobspec,
         detail::dfu_impl_t::update ();
     } else if ( (rc = schedule (jobspec, meta, x, op, root, dfv)) ==  0) {
         *at = meta.at;
-        if (*at < 0 or *at >= graph_end) {
+        if (*at == graph_end) {
+            detail::dfu_impl_t::reset_exclusive_resource_types
+                                                        (exclusive_types);
+            // no schedulable point found even at the end of the time, return EBUSY
+            errno = EBUSY;
+            return -1;
+        }
+        if (*at < 0 or *at > graph_end) {
             detail::dfu_impl_t::reset_exclusive_resource_types
                                                         (exclusive_types);
             errno = EINVAL;