Previously when a ClientHello arrives with a valid cookie using
DTLSv1_listen() we only "peeked" at the message and left it on the
underlying fd. This works fine for single threaded applications but for
multi-threaded apps this does not work since the fd is typically reused for
the server thread, while a new fd is created and connected for the client.
By "peeking" we leave the message on the server fd, and consequently we
think we've received another valid ClientHello and so we create yet another
fd for the client, and so on until we run out of fds.

In this new approach we remove the ClientHello and buffer it in the SSL
object.

Fixes #6934
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/7375)