Identify required patches for Envoy Windows port public alpha

[wrowe]

This is a placeholder to tag all WIP to get us to a usable alpha state.

Reflects another milestone in the history of issue #129

[wrowe]

#10871 "Replace pipe() with os_sys_calls_impl::socketpair(AF_UNIX,...) in buffer tests"
offers a suggestion of applying the evutil_win_socketpair_afunix() helper of libevent.

[wrowe]

#11655 doesn't seem critical for alpha, but should probably be implemented before the first beta candidate.

[wrowe]

Open question of whether #10420 is needed before alpha.

[wrowe]

cc @envoyproxy/windows-dev

[wrowe]

cc @rmiller14

[sunjayBhatia]

Closing this, alpha has been announced, any issues for Windows beta should be added to this milestone: https://github.com/envoyproxy/envoy/milestone/16