Continue implementation of short tunnel build messages (proposal 157)

- Generate and parse short record format
- Encrypt and decrypt short records
- Register handlers for 3 new messages ITBM/STBM/OTBRM
- Send ITBM/STBM if all hops support it (disabled)
- Reply with OTBRM at OBEP if STBM received (disabled)
- Send STBM at IBGW if ITBM received (disabled)
- Add logic for when to send new messages
- ChaCha encryption of other short records
- Fix compare logic in ITBM parser (ticket #2814)

All is still preliminary, disabled, untested; proposal is still incomplete

Still todo:
- Fill in plaintext record for ITBM/OTBRM
- OTBRM key/tag