Provide functions to write early data

We provide SSL_write_early() which *must* be called first on a connection
(prior to any other IO function including SSL_connect()/SSL_do_handshake()).
Also SSL_write_early_finish() which signals the end of early data.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2737)