Add support for client side parsing of the PSK extension

Requires a refactor of the ServerHello parsing, so that we parse first and
then subsequently process. This is because the resumption information is
held in the extensions block which is parsed last - but we need to know that
information earlier.

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