MEDIUM: ssl: Handle early data with OpenSSL 1.1.1

MEDIUM: ssl: Handle early data with OpenSSL 1.1.1

When compiled with Openssl >= 1.1.1, before attempting to do the handshake,
try to read any early data. If any early data is present, then we'll create
the session, read the data, and handle the request before we're doing the
handshake.

For this, we add a new connection flag, CO_FL_EARLY_SSL_HS, which is not
part of the CO_FL_HANDSHAKE set, allowing to proceed with a session even
before an SSL handshake is completed.

As early data do have security implication, we let the origin server know
the request comes from early data by adding the "Early-Data" header, as
specified in this draft from the HTTP working group :

    https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-replay