Make nsfw context aware

[pixtron]

Currently the nsfw module is currently not context aware. This prevents from using nsfw in multiple threads (nodejs/nan/pull/792).

Also electron deprecated "Loading non context-aware native modules" (electron/electron/issues/18397)

Steps to reproduce

Running below script with node v12 should work, and emit each change event twice (once from main, once from worker)

const { Worker, isMainThread } = require('worker_threads');
const nsfw = require('nsfw');
const homedir = require('os').homedir();

if (isMainThread) {
  new Worker(__filename);
} else {
  console.log('Inside Worker!');
}

nsfw(homedir, function(events) {
  console.log(`Got event in ${isMainThread ? 'Main' : 'Worker'}`, events)
}).then(function(watcher) {
  return watcher.start();
});

The error thrown is:

events.js:288
      throw er; // Unhandled 'error' event
      ^
Error: Module did not self-register: '/.../nsfw/build/Release/nsfw.node'.
    at Object.Module._extensions..node (internal/modules/cjs/loader.js:1208:18)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:901:14)
    at Module.require (internal/modules/cjs/loader.js:1044:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.<anonymous> (/.../nsfw/lib/src/index.js:3:16)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:901:14)
Emitted 'error' event on Worker instance at:
    at Worker.[kOnErrorMessage] (internal/worker.js:204:10)
    at Worker.[kOnMessage] (internal/worker.js:214:37)
    at MessagePort.<anonymous> (internal/worker.js:141:57)
    at MessagePort.emit (events.js:311:20)
    at MessagePort.onmessage (internal/worker/io.js:78:8)

[implausible]

Thanks for the report! The direction I want to take this module in is to refactor for N-API.

[blarfoon]

Is there a roadmap or eta for this?

[implausible]

@killpowa When this #104 lands. So probably this week.