Implement curl_multi_await()

Summary: Push curl sockets into s_asio_event_base loop
along with optional timeout.  Trigger awaitable completion
on any handle activity.

Reviewed By: @JoelMarcey

Differential Revision: D1700978

Signature: t1:1700978:1417563769:046272e19366f80a118c552ce64c7cff7573da4d