Improve performance of RedisInterface#make_list_length (#509)

This implementation will only execute 1 to Redis command, instead of
2n (n being the total number of elements to trim from the list).

The operation will still take O(n) time, but the constants will be
much better, since only 1 network round trip is required.

Author: mlovic

lib/split/redis_interface.rb

@@ -35,9 +35,7 @@ def remove_last_item_from_list(list_name)
     end
 
     def make_list_length(list_name, new_length)
-      while list_length(list_name) > new_length
-        remove_last_item_from_list(list_name)
-      end
+      redis.ltrim(list_name, 0, new_length - 1)
     end
 
     def add_to_set(set_name, value)