Fix best_inner_indexscan to return both the cheapest-total-cost and
cheapest-startup-cost innerjoin indexscans, and make joinpath.c consider both of these (when different) as the inside of a nestloop join. The original design was based on the assumption that indexscan paths always have negligible startup cost, and so total cost is the only important figure of merit; an assumption that's obviously broken by bitmap indexscans. This oversight could lead to choosing poor plans in cases where fast-start behavior is more important than total cost, such as LIMIT and IN queries. 8.1-vintage brain fade exposed by an example from Chuck D.
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