Simplify new session ticket encoding/generation.

Simplify new session ticket encoding/generation.

The original code did a crazy encode/malloc/encode/decode/modify/encode
dance, in order to encode a session in the form needed to encrypt then add
to a session ticket. By modifying the encoding functions slightly, we can
do this entire dance as a single encode.

Inspired by similar changes in BoringSSL.

ok inoguchi@ tb@