Summary:
The folly::coro::Task coroutine type now captures the current RequestContext when the coroutine suspends and restores it when it later resumes.

This means that folly::coro::Task can now be used safely with RequestContext and RequestContextScopeGuard.

Reviewed By: andriigrynenko

Differential Revision: D9973428

fbshipit-source-id: 41ea54baf334f0af3dd46ceb32465580f06fb37e