use zsync to update offline routing data

[Martin Kolman]

• use zsync to update routing data from the server
• like this, only changes would be transferred, making frequent routing data updates feasible
  • packs for large countries can take up multiple GBs

• zsync: ​http://zsync.moria.org.uk/

• drawbacks:
• needs to store two copies of the routing data
  • one archive for direct download
  • one in uncompressed form + zsync control file for updates only
    • needs zsync files for all files
    • on the other hand as zsync won't overwrite the file until the download is finished, working on multiple smaller files helps