Use opslog instead of counters

Account for eventual consistency by storing log items for
create/update/delete operations instead of using counter keys. We can
then map/reduce over the log items in order to extract category sizes
and object counts. Furthermore, we can combine single items from time to
time in order to keep things fast and tidy.

spec/spec_helper.rb

@@ -58,7 +58,7 @@ if app.settings.riak
   end
 
   def purge_all_buckets
-    [data_bucket, directory_bucket, auth_bucket, binary_bucket].each do |bucket|
+    [data_bucket, directory_bucket, auth_bucket, binary_bucket, opslog_bucket].each do |bucket|
       bucket.keys.each {|key| bucket.delete key}
     end
   end