plugin: use plugin_factors map to fetch weights

Instead of hard-coding the weights for all of the
priority factors in the priority_calculation() function,
fetch the weights stored in the plugin_factors map, which
contains information about the various priority factors and
their associated weights defined in the flux-accounting DB
plugin_factor_table. If for some reason the weight of either
factor cannot be found, a weight of 0 will be used in the
priority calculation of a job.

src/plugins/mf_priority.cpp

@@ -85,8 +85,9 @@ int64_t priority_calculation (flux_plugin_t *p,
     int fshare_weight, queue_weight;
     struct bank_info *b;
 
-    fshare_weight = 100000;
-    queue_weight = 10000;
+    // fetch associated weights of all of the priority factors
+    fshare_weight = plugin_factors["fairshare"];
+    queue_weight = plugin_factors["queue"];
 
     if (urgency == FLUX_JOB_URGENCY_HOLD)
         return FLUX_JOB_PRIORITY_MIN;