[Tile-serving] [openstreetmap/osm2pgsql] Make sure cache is freed before starting indexing (#813)

[Hartmut Holzgraefe]

On 13.01.2018 22:32, Sarah Hoffmann wrote:
> The pending processor holds shared_ptr references to the node caches. 
> That prevents the caches from being freed when mid::stop() is called. 
> This change makes sure the pending processor goes out of scope before 
> indexing starts.
> 
> @StyXman <https://github.com/styxman> This resolves the problem with not 
> freeing cache memory. Turned out to be a completely different issue. So 
> no further changes for the stop() function needed.

\o/