Envoy hot restart

> (In the current implementation Envoy does not make use of the SO_REUSEPORT socket option. This is primarily historical since that socket option is only available on relatively new kernels. At some point support for old kernels can be dropped and we will switch the code to using this socket option)

FWIW: Using SO_REUSEPORT is still problematic (in Linux) under high traffic, it could potentially drop connections with RST. So if Envoy is running as edge proxy, I could imagine it running into that (though in Envoy, one could avoid hotrestarts for lot of config changes, so the need to hot restart is not that often …).

To properly support hotrestarts, HAProxy actually fell back to transferring listen sockets over UDS, just like Envoy does currently: https://www.haproxy.com/blog/truly-seamless-reloads-with-haproxy-no-more-hacks/

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