Simplified and fixed estimate computation.

Thsi patch greatly simplifies the estimate computation. To that end, it also simplifies the compiler hints.

The prior version was more expressive, but was never used because it required the programmer to ahve detailed knowledge about the program (like per column group cardinalities).

In the long run, we need a clear plan how to best expose hints. Until then, I think this makes it much simpler to use and it fixed a buch of bugs on the way.