Title: Envoy is spinning CPU on Windows

Description:

In 2019 version of Windows Server there is no support for level-based events which causes Envoy to spin CPU to process "empty" write ready events.

To resolve this issue we consider the following three approaches:

1. Selectively turn on and off the level based events. The idea is to only listen to file event notifications when we want to write to the socket (either because the previous write call got blocked or new data came in).
2. Change the socket implementation to use the I/O completion ports paradigm.
3. Rely on a different backend (see libevent replacement with libev #4952)

cc: @envoyproxy/windows-dev