Don't use Pthread in EventBase

Don't use Pthread in EventBase

Summary:
Pthread is currently a dependency of Folly that is not really necessary on Windows, or even with standard C++ for the most part, so start work on killing it in Folly.
This switches EventBase to using `std::thread::id`'s instead, which also means we aren't reliant on the implementation detail that thread id 0 is invalid. Well, we are, but it's now the standard library's fault not ours.

Reviewed By: yfeldblum

Differential Revision: D4418128

fbshipit-source-id: a9c95ac6c7305c960156a4ad684b6db89b5856d9