resource: set default job duration based on current time left

Problem: When a job is submitted without a duration specified and
the current graph has a limited duration, the jobspec duration is set
to the graph duration. This is not ideal for any job submitted after
the first instant of the graph's lifetime, since the job's expiration
will be set after the instance has expired.

Ideally, jobs without a specified duration should inherit the instance
expiration not its duration. Do the next best thing and set the
duration to the instance expiration (graph_end) - now, which should
result in the expiration of the job matching that of the instance.

Fixes #1103

resource/traversers/dfu_impl.hpp

@@ -70,8 +70,9 @@ struct jobmeta_t {
         now = t;
         jobid = id;
         alloc_type = alloc;
+        const auto now = std::chrono::system_clock::now();
         int64_t g_duration = std::chrono::duration_cast<std::chrono::seconds>
-            (graph_duration.graph_end - graph_duration.graph_start).count ();
+            (graph_duration.graph_end - now).count ();
 
         if (g_duration <= 0) {
             errno = EINVAL;