fixes #3117

I started with this on the side, working on it here and there. to re-iterate: we want to be able to load other tile extracts while a consumer application is running (e.g. valhalla_service). in #3117 @kevinkreiser proposed to use a fixed-width index file storing the tile_id and byte offset in the final tar file which the graphreader can read first (if available) to really fast initialize the tile_extract_t (faster because it does not have to scan through the entire tar file thanks to the index file).

tasks:

• set up python script
• tests python (needs some refactoring)
• adapt graphreader to read the index file first if present
• tests graphreader
• some open questions:
  • this (currently) does not work for existing extracts, so far it only builds an extract from mjolnir.tile_dir. is that fine with you? we could also add that ability and it sorta was what @kevinkreiser initially suggested -> not necessary IMO
  • where to add this functionality: python bindings for sure IMO, what about valhalla_service? -> subsequent PRs

@kevinkreiser I didn't do much yet on the c++ side, will comment a bit on the files, a few pointers would be cool:)

example commandline: python scripts/valhalla_build_extract --config site/valhalla.json