[Tile-serving] [osm2pgsql] Use GeoHash ordering instead of way ordering (#242)

[cquest]

Interesting and strange result at the same time.
ST_Geohash uses a loop and I don't understand why less iterations could take more time !

I did a quick test yesterday on a small extract (to limit other side effects like I/O) and the 6 chars ST_Geohash were faster than the 8 chars one in my test. My test was not the same, I simply created a Geohash based index later used by CLUSTER.

Maybe a side effect on the CREATE TABLE / SELECT / ORDER BY due to much more similar values ?

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https://github.com/openstreetmap/osm2pgsql/pull/242#issuecomment-68341077
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