fast path for mongodb batches (otherwise it's O(n^2) memory, which gets ugly fast)

src/batch.js

@@ -23,6 +23,11 @@ var async = require('async'),
 			return callback(new Error('[[error:process-not-a-function]]'));
 		}
 
+		// use the fast path if possible
+		if (db.processSortedSet && typeof options.doneIf !== 'function' && !utils.isNumber(options.alwaysStartAt)) {
+			return db.processSortedSet(setKey, process, options.batch || DEFAULT_BATCH_SIZE, callback);
+		}
+
 		// custom done condition
 		options.doneIf = typeof options.doneIf === 'function' ? options.doneIf : function(){};
 
@@ -58,4 +63,4 @@ var async = require('async'),
 		);
 	};
 
-}(exports));
+}(exports));