[performance] Added FastCGI support

[mmd-osm]

• Adds FastCGI support
• Adding new configure parameter --enable-fastcgi - new code is optional and as least invasive as possible.
• Resulting binary still supports both CGI and FastCGI
• Added a new Index Cache, which caches data_index & map_index across several queries. Once replicate_id changes, the cache is being discarded
• Other caches (like User_Data_Cache and relation_member_roles) are treated in a similar way and discarded upon replicate_id changes.

See #209 for more details on how to configure Apache / Nginx for FastCGI.

TODOs:

• Review coding
• Check if there's anything new in 0.7.54 which needs to be initialized as well after each query
• More testing!
• setrlimit needs to be changed to soft limits, otherwise [maxelem: ...] and [timeout: ...] cannot be set/updated across several queries
• Use of std::lock_guard<std::mutex> guard(transaction_mutex); is needed as preparation for [performance] Parallel database update #384, it's not really needed for FastCGI itself.

Thanks for reviewing!

[DaMatrix]

Any updates on this?

[mmd-osm]

Moved to https://github.com/mmd-osm/Overpass-API