Improve Join expression constant folding

[frankmcsherry]

Our join expression constant folding bails out if it sees a candidate number of rows that exceeds a threshold. However, it does this testing as it goes, before it applies equality constraints. Applying the equality constraints as one goes could dramatically reduce the number of candidates, and generally make the process faster as well as less likely to fail out.

[aalexandrov]

I will pick this up next. If I read this correctly the right entry point for changing is this TODO comment.