Closes SUR-53
When creating a new context for subqueries or statement clauses, we used to have to clone any variables/values, and freeze the context, so that it could be used across threads and async boundaries.
Now with the new executor pattern for parallel queries, we can pass references instead, improving performance by removing unnecessary cloning of values.
Previously the expression operations happened right-to-left so `1 - 2 - 3 - 4 - 5` was actually calculated as `(1 - (2 - (3 - (4 - (5)))))`. Now the expressions are calculated according to BODMAS, with boolean expressions calculated last.
