Skew Join (someone please implement)

[johnynek]

Here is what we do:

1. sample the left and right side way down (like 1/1000, but probably a parameter of the skewJoin).
2. do a join of the left and right (maybe hashjoin if inner join is okay).
3. esimate the number of items on the left and right for each key.
4. set a replication factor for each key for each side, such that every (key, replication) bucket has about the same number of items.
5. leftJoinWithTiny this replication factor for each key, if there is no replication factor choose 1.
6. adapt blockJoin to read the key replication from the pipe, so that we can use different replications for each key.
7. update Matrix API to have skew hinting, and if it is set, use skewJoin in matrix product.
8. Profit.

[johnynek]

Done: #229