Refactor(optimizer): Optimize USING expansion

[VaggelisD]

This PR is an optimization follow-up on #4113

When processing multiple joins such as A JOIN B JOIN C USING(col), we gather the columns from the previous sources (A, B etc) to expand USING (col) to ON c.col = <source>.col. Previously, this operation would happen from scratch on each iteration, which can be avoided. A few key observations:

1. The ordered list is constructed as a subset of scope.selected_sources; It can have [0, N] elements before the join loop
2. At each iteration, we'll add the current join_table to ordered; It is implied that join_table is a member of scope.selected_sources as it participates in the scope's JOIN clause(s) so (1) continues to hold

The combination of (1) and (2) means that we can avoid checking if source_name is both in scope.selected_sources and ordered; Besides that, (2) drives incremental construction of columns at each iteration.

This PR also adds one more test case of a longer JOIN chain; This is to ensure that columns from all intermediate join tables are gathered, even if there is no USING clause.