Simplify session ticket parsing/handling.

Simplify session ticket parsing/handling.

The original implementation is rather crazy and means that we effectively
have two lots of code that parse a ClientHello and two lots of code that
parse TLS extensions. Partially simplify this by passing a CBS containing
the extension block through to the session handling functions, removing the
need to reimplement the ClientHello parsing.

While here standarise on naming for session_id and session_id_len.

ok inoguchi@ tb@