Async Middlewares support + Perf improvements

Thanks to @Akkuma contribution in #49, we are now supporting async middlewares, so developers can control modern execution flows as described below:

service.use(async(req, res, next) => {
  await next()
  console.log('Next middlewares were executed!')
})

service.use(...)

Also capturing exceptions inside async middlewares becomes easier:

service.use(async(req, res, next) => {
  try {
    await next()
  } catch(err) {
    res.send(err)
  }
})

On top of this, restana becomes also 1 - 4 % faster while dealing with middlewares. 🚀🚀🚀