Add a capability for TLSProxy to wait for a session before killing s_client

TLSProxy normally fires off s_client, which creates a connection to the
server. TLSProxy also pipes some data to send to the process and s_client
automatically exits when the pipe hits eof. Unfortunately this means that
it sends the data and closes before it has processed the NewSessionTicket
returned from the server in TLSv1.3. This commits adds an option for
s_client to stay loaded until the sesion has been processed. A side effect
of this is that s_client never sends a close_notify in this mode, so we
count success as seeing that data has been transferred.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)