WIP: server: multicore

[missinglink]

DO NOT MERGE

This PR enables multicore support for the interpolation service.

I do not think this should be merged in its current state due to:

each process loads its own copy of 'node-postal', so each process will consume ~2GB of RAM.

We could consider defaulting to 1 process instead of os.cpus().length and then allow ops engineers to modify the setup to suit their environment using the CPUS env var.

Thoughts?

[dianashk]

I think until we switch to using libpostal as a service we shouldn't merge this. It would just complicate things and we'd get more support questions than benefits.

[dianashk]

Closing for the time being.

[orangejulius]

We definitely should re-evaluate this after #146 is merged!