Allow create_index_paths() to consider multiple join bitmapscan paths.

In the initial cut at the "parameterized paths" feature, I'd simplified
create_index_paths() to the point where it would only generate a single
parameterized bitmap path per relation.  Experimentation with an example
supplied by Josh Berkus convinces me that that's not good enough: we really
need to consider a bitmap path for each possible outer relation.  Otherwise
we have regressions relative to pre-9.2 versions, in which the planner
picks a plain indexscan where it should have used a bitmap scan in queries
involving three or more tables.  Indeed, after fixing this, several queries
in the regression tests show improved plans as a result of using bitmap not
plain indexscans.