Adding pluggablility to work allocation, along with a new work allocation strategy to group small topics within the same task.

The avg size puts the 1/2 of the items as small items, would this bar too high?  Especially we put all the partitions belong to the same topic into small group, so it's more than 1/2.  Should the threshold set to 1/3 or 1/4 of the total items?