Follow-ups now that the MVP of the feature is merged (#2298).

• Merge async payments spec
• Merge static invoice server bLIP

Async receive:

• Fix rare race documented in Onion Message Mailbox #2950
• Update async offers/static invoices when a channel closes/opens/fees change
• Support the static invoice server configuring how many invoices they'll store for the user
• Move more code from flow.rs into async_receive_offer_cache.rs
• Consider supporting async receive offers for a specified amount

Async send:

• Trampoline #2299 integration
  • Support shortening the HTLC’s CLTV at sending-time
• Don't FC during startup chain relay if we might be able to salvage the channel by connecting to a peer #4048
• Support multi-hop blinded paths for ReleaseHeldHtlc #4066
• Support manual handling for static invoices
• Retry held_htlc_available messages
• Generate event when an async payment is pending (see Support paying static invoices #3140 (comment) )
• Add RecentPaymentDetails variant for async payments (see Support paying static invoices #3140 (comment) )