Avoid recreating all metrics on reload

This commit fixes a bug in the comparison of the loaded metrics
and all application metrics. Because the Props for each metric
differ in the output of couch_stats:list/0 and
load_metrics_for_application/0 the set subtraction operations
in reload_metrics/0 would always leave a full set, so all metrics
would be deleted and created on every reload.

This is fixed by ensuring the sets for existing and current
metrics consist of {Name, Type} only.