Start middle table indexing once processing is finished

[pnorman]

With in-memory pending, the middle tables are only read from once processing is finished. This allows us to build indexes right away, including the GIN index on planet_osm_ways.nodes, which is often slower than the rest of the import.

By starting it sooner, it will make the pending ways and other indexes no longer part of the critical path for import speed on a --slim import. It might increase contention for disk, but this is probably not an issue.

Not planned for resolution pre-merge