cache sql.Schema in plan.Project and plan.TableAlias

[jycor]

Changes:

• plan.Project.Schema() and plan.TableAlias.Schema() create a new sql.Schema every time they are called, which can be a lot during analysis. So, the results are cached.
• sql.Index.Expressions() retrieves a new []string, so we should cache that as well. Additionally, many times we unqualify the results, so I added a UnqualifiedExpressions() to avoid doing this extra processing every time.
• changes idxExprsColumns to avoid using strings.Split to avoid extra allocations