Choose to use std::async for now to implement multi-threaded compiles.
This should work across platforms and also be usable for other
threading tasks. Note that std::async does not have a good way to
wait for multiple std::futures. Also the Linux compile of std::async
is broken due to a bug in an old STL version, so disable it on this
platform.

The implementation uses a static polymorphism approach, which should
have very good performance (no virtual calls).

This design leaves the door open for other future implementations,
such as a Win32 thread pool, or one based on angle::Platform.

BUG=angleproject:422

Change-Id: Ia2f13c3af0339efaca1d19b40b3e08ecca61b8e8
Reviewed-on: https://chromium-review.googlesource.com/413712
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>