Fix bug where we double-pay an offer due to stale manager

Fix bug where we double-pay an offer due to stale manager

This fixes the following bug:
- An outbound payment is AwaitingInvoice
- We receive an invoice and lock the HTLCs into the relevant ChannelMonitors
- The monitors are successfully persisted, but the ChannelManager fails to
  persist, so the outbound payment remains AwaitingInvoice
- We restart, causing the channels to close due to a stale ChannelManager
- We receive a duplicate invoice, and attempt to pay it again due to the
  payment still being AwaitingInvoice in the stale ChannelManager

After the fix for this, we will notice that the payment is already locked into
the monitor on startup and transition the incorrectly-AwaitingInvoice payment
to Retryable, which prevents double-paying on duplicate invoice receipt.