Lagrang3 points out that if we hit a maximum, we should take into account
the reserve. This is true, but it's hard for the caller to do, so change
the API to be slightly higher level.
Tell "inform" what happened, and it adjust the constraints appropriately.
This makes the least assumptions possible (a reserve does *not* mean that
the capacity was actually used at that time).
We also add a mode to say "this succeeded": for now this does nothing,
but it could reduce both min/max capacities, and add capacity in the
other direction. This is useful for future payments, but not as useful
for the current one.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
