Introduce health check #239

To observe the uptime of obyte nodes, a health check would be great

See e.g.
https://medium.com/better-programming/how-to-add-a-health-check-to-your-node-js-app-5154d13b969e

Remarks:

• healthcheck might be different for e.g. witness / hub / ... (what it internally checks)
• would be good preparation for later containerization / dockerization of obyte nodes