multicore http server

[missinglink]

This draft PR is #249 plus a single commit to add multi-core support to the http server via the cluster module.

When using the node-postal 'module' adapter it is recommended to set the env var CPUS=1 to force the server single-threaded.

[missinglink]

@orangejulius I did some basic testing on this and found that using multicore and libpostal service resulted in worse performance than running it single-threaded using the npm module.

Maybe there's an error in how I'm using the async code or something else going on?

[orangejulius]

Yeah, I recall experiencing and reading about poor performance from the cluster2 module in the past.

After all, it's essentially a load balancing layer where different Node.js processes are communicating with each other. I imagine it's not nearly as efficient as say, Nginx in front of several identical Node.js processes reading from the same SQLite DB.