Global event loop for asio socket events

Summary: Basic folly::EventBase running in a separate thread
for use by awaitables which need to wait on socket events.

See curl_multi_await() for an example usage.

Reviewed By: @markw65

Differential Revision: D1700900

Signature: t1:1700900:1417561060:43a58182b0342f8b630c2de7faada4bbb0bda484