Add logging of cache ratio

Two reasons for logging this:

- to assist in sizing the ledger cache;
- to resolve the mystery when there appear to be no fetches from the penciller (as the penciller does not report fetches from the ledger cache)

src/leveled_log.erl

@@ -80,6 +80,8 @@
     {"B0020",
         {warn, "Ratio of penciller cache size ~w to bookie's memory "
                     ++ "cache size ~w is larger than expected"}},
+    {"B0021",
+        {info, "Bookie fetch RequestCount=~w and CacheCount=~w"}},
 
     {"R0001",
         {debug, "Object fold to process batch of ~w objects"}},