Async Payments

[valentinewallace]

Async-receive:

• Support being the async receiver’s channel counterparty (eg LSP) per the spec PR
  • Support store-and-forward mailbox for onion messages Onion Message Mailbox #2950
  • Track what users we’re waiting to come online (+ persist across restarts)
  • Notify sender via OM when receiver comes online
• Support being the always-online (for now) sender
  • Notify the receiver’s LSP that an HTLC is being held for the receiver
  • Send HTLC when notified by the receiver’s LSP
• Support being the async receiver
  • Support receiving keysend payments to blinded paths Support keysend to blinded paths #2935, Verify blinded keysend PaymentSecrets #2956
  • Support creating a AP BOLT12 offer + keysend invoice that signals that the recipient is often-offline + their LSP’s contact info
  • Spec the above Support async payments in BOLT 12 lightning/bolts#1149

Async-send:

• Trampoline Trampoline #2299
• Parse AP BOLT12 invoices
• Support sending an async HTLC and corresponding OM
• Support retrying the OM or spec out+implement an ACK OM