resolves dbt-labs#150

Problem
The delete query for the 'delete+insert' incremental_strategy with 2+ unique_key columns is VERY inefficient. In many cases, it will hang and never return for deleting small amounts of data (<100K rows).

Solution
Improve the query by switching to a much more efficient delete strategy:

```
delete from table1
where (col1, col2) in (
    select distinct col1, col2 from table1_tmp
)
```