[Tile-serving] [openstreetmap/osm2pgsql] Wrong the rule of thumb in slim mode (#1431)

[Valerij Ivashchenko]

According to the [docs](https://osm2pgsql.org/doc/manual.html#caching):
```
The rule of thumb in slim mode is as follows: use the size of the PBF file you are trying to import or about 75% of RAM, whatever is smaller.
```

But in fact seems that cache size must be above twice lager then source file.
I've made tests on 2 pbf-files and it shown that for cache size equal to source file size import speed for ways just 30-40% from max possible.

![image](https://user-images.githubusercontent.com/603401/111852498-3670ee80-8928-11eb-8ff3-4be8090798d7.png)

![image](https://user-images.githubusercontent.com/603401/111852512-44267400-8928-11eb-9033-60eea746a200.png)

[Google sheet with tests results](https://docs.google.com/spreadsheets/d/1IjCndMOyr6cM2FDRu53ASZdIR6ZdmUeqoREf-V9TGGk/edit?usp=sharing).






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