Summary:
Add a generic function for performing a blocking-wait on any awaitable object.

This obsoletes the use of the `.wait()` and `.get()` methods on  `folly::coro::Future<T>` and so these methods have now been removed.

Reviewed By: andriigrynenko

Differential Revision: D9384837

fbshipit-source-id: bcff5b348c024a3fc9337c67d996c447e0723fcc