Summary:
OpenSSL 1.1.0 uses a fiber-based (makecontext/swapcontext) API to do
asynchronous operations. When some operation deep inside the stack calls
ASYNC_pause_job, SSL_accept returns -1 with error SSL_ERROR_WANT_ASYNC.
OpenSSL chose to use fds to wait on, so after SSL_accept returns, we create an
AsyncPipeReader to restart SSL_accept when the pipe becomes readable, which is our
indication that the async job processing has finished.

Also implemented a test to kick off an async job in a different thread that creates a pipe
and gives the read end back to the SSL* before calling ASYNC_pause_job

Reviewed By: yfeldblum

Differential Revision: D5977514

fbshipit-source-id: 3aba2e45b9357dc28cf7cf785654072f8ba8dd65