Users can send private offers that will only be seen by one address.
To send a private offer:
1. recipient creates a new address to receive offers on
2. recipient sends the pubkey for the newly created address to the offerer
3. offerer imports the recipient's pubkey
4. offerer sends a new offer to the recipients key instead of the public network
Nodes will ignore offers sent on keys other than the network key or keys created for offer-receiving.
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