Make HMR heartbeat configurable

[majelbstoat]

When running NextJS inside a docker container that mounts a local volume, the server is typically slower to respond than on bare metal. In combination with Node 8, this often leads to INCOMPLETE_CHUNKED_ENCODING errors when hot module reloading.

This was fixed for bare metal setups in #2166, but the chosen heartbeat is often too short for a containerised instance.

Can we make heartbeat in server/hot-reloader.js configurable through next.config.js?

https://github.com/zeit/next.js/blob/canary/server/hot-reloader.js#L224

[timneutkens]

As no one else has brought up or upvoted this issue I don't think we should change the default.

[majelbstoat]

If anyone else is struggling with this, the problem went away when I started using http2 in dev.