Parallelize interpolation build #264

This issue is to record the fact that the process of building the interpolation database is single threaded and can be quite slow.

For a full planet build you can expect it to take 6+ days. We recommend avoiding setting up the interpolation database when initially experimenting with Pelias to save yourself some time.

We welcome efforts to parallelize it, but beware: it's not as easy as it sounds. Feel free to shoot an email to team@pelias.io if you are interested in helping out.

Over time we may add more details here about the architecture of the interpolation service and why it's single threaded :)