[Tile-serving] [openstreetmap/osm2pgsql] Skip transforms for untagged objects (#706)

[Paul Norman]

>From ClearTables/ClearTables#109

A full featured multi-transform can easily contain 10+ point tables. Reasonably written transforms will exit in tens or hundreds of ns, but the cost of calling 10 transforms adds up for 4b nodes. 97% of nodes have no tags and will be filtered out by any transform I can think of.

Skipping these untagged nodes in C++ would be faster than calling to Lua and skipping them. Although the performance is mainly an issue with the multi-backend, I propose also doing it for the pgsql backend with lua transforms. The C backend already skips untagged nodes under all possible style files, but it's worth checking that it skips it early in the transform to avoid unnecessary work.

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