[Tile-serving] [osm2pgsql] Reduce memory usage of node cache (#360)

[Paul Norman]

The import which failed before works now.


```
...
node cache: stored: 1484575472(54.67%), storage efficiency: 94.39% (dense blocks: 1406958, sparse nodes: 66070009), hit rate: -114.53%

Osm2pgsql took 33536s overall
```

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/pull/360#issuecomment-116841967
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