[Events] Add an file event wakeup method with socketpair

Description:

The goal is to create a premitive in source/common/event that wakes up (schedules) an event using a socketpair instead of scheduleCallbackCurrentIteration. The reason for that is that scheduleCallbackCurrentIteration is using event_active(&raw_event_, EV_TIMEOUT, 0); which takes a lock.

See comment in: https://github.com/envoyproxy/envoy/pull/13954#discussion_r533796896

This mechanism could be applied to:

1. Win32 DirectoryWatcher
2. Replace the wakeup mechanism in dispatcher_impl.cc -> void DispatcherImpl::post(std::function<void()> callback)

For Win32 it is good to also have #10871 if we are going to build on top of socketpairs