Add a heartbeat mechanism, and timeouts support

[dunglas]

Some reverse proxies kill connections after a (short) period of inactivity. Also, sometimes it's necessary to kill connections after some time to cleanup the memory.

This PR introduces new environment variables to start a heartbeat mechanism (that will prevent reverse proxies and some old browsers to kill the connection), and as well as read and write timeouts.

They are all disabled by default.

• HEARTBEAT_INTERVAL: interval between heartbeats (useful with some proxies, and old browsers), set to 0s to disable (default), example 15s
• READ_TIMEOUT: maximum duration for reading the entire request, including the body, set to 0 to disable (default), example: 2m
• WRITE_TIMEOUT: maximum duration before timing out writes of the response, set to 0 to disable (default), example: 2m

[karser]

@dunglas what do you think of setting HEARTBEAT_INTERVAL=30s by default? Heartbeat is required for both native and polyfill event source implementations.

Update: I see, the native EventSource uses keep-alive

[dunglas]

It sounds good to me!