fix(proxy): close proxy when server closes

[chimurai]

fixes #189
fixes #287

Example Graceful Shutdown (source: https://expressjs.com):

process.on('SIGTERM', () => {
  debug('SIGTERM signal received: closing HTTP server')
  server.close(() => {
    debug('HTTP server closed')
  })
})

More on graceful server shutdown: https://expressjs.com/en/advanced/healthcheck-graceful-shutdown.html

[coveralls]

Coverage decreased (-0.7%) to 87.46% when pulling 32687ee on proxy-close into 7c3fd07 on master.