Some debug stats are counted under the wrong metrics

[JosiahWI]

The one I noticed is in agg_copy; in debug mode, every time bytes are copied to the aggregation buffer to prepare for writing, it's counted as a backlog failure instead of a write. The cache will appear to have a problem due to a high number of incorrectly reported backlog failures and no bytes written. This was introduced in f23826d; I don't know whether there might be other similar issues introduced in this manner.

-        CACHE_DEBUG_SUM_DYN_STAT(cache_write_bytes_stat, vc->write_len);
+
+// ToDo: Why are these for debug only ?
+#ifdef DEBUG
+        Metrics::increment(cache_rsb.write_backlog_failure);
+        Metrics::increment(vol->cache_vol->vol_rsb.write_backlog_failure);
+#endif