BIP75 Simplification and Enhancements

This update for BIP75 makes the following changes:

• Remove duplicate, encrypted versions of each Payment Protocol message
• Add encapsulating messages that allow for both plaintext and encrypted messaging as well as status messaging within the protocol
• Change the AES-256 mode to GCM (from CBC), and including status_code || status_message as Additional Authenticated Data in the GCM cipher
• Update use of ECDH X-point to instead use SHA256(ECDH().x) to match up with the libsecp256k1 implementation of ECDH)

These changes aim to make the protocol much easier to implement on both the client and store & forward server sides.