sql.Rewrite refactor

[araddon]

Complete re-write of the sql.ReWrite() which is crap code.

Goals:

• Allow sql.ReWrite to support push-down to different data sources when using multi-source queries (joins, select where in, etc).
• Allow poly-fill missing feautures (ie, group-by in cassandra) but support partial push-down.
• Allow function re-write (ie, postgres, mysql, etc have different names for the same functions). Allow a dialect-writer to support function re-write.

[araddon]

plan

• [ ] much better unit tests on the sql-rewrite, and more component oriented
• [ ] allow function or node-level rewrite for different dialects