Increase the max size limit for a CertificateRequest message

Previous versions of OpenSSL had the max size limit for a CertificateRequest
message as |s->max_cert_list|. Previously master had it to be
SSL3_RT_MAX_PLAIN_LENGTH. However these messages can get quite long if a
server is configured with a long list of acceptable CA names. Therefore
the size limit has been increased to be consistent with previous versions.

RT#4198

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>