Factor out the TLSv1.3 code that handles content from TLS records.

Factor out the TLSv1.3 code that handles content from TLS records.

Currently, the plaintext content from opened TLS records is handled via
the rbuf code in the TLSv1.3 record layer. Factor this out and provide a
separate struct tls_content, which knows how to track and manipulate the
content.

This makes the TLSv1.3 code cleaner, however it will also soon also be used
to untangle parts of the legacy record layer.

ok beck@ tb@