We need to stash/save the amount of the lease fees on a leased channel,
we do this by re-using the 'push' amount field on channel (which is
technically correct, since we're essentially pushing the fee amount to
the peer).
Also updates a bit of how the pushes are accounted for (pushed to now
has an event; their channel will open at zero but then they'll
immediately register a push event).
Leases fees are treated exactly the same as pushes, except labeled
differently.
Required adding a 'lease_fee' field to the inflights so we keep track of
the fee for the lease until the open happens.
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