It was weird not to have a capacity associated with localmods channels, and
fixing it has some very nice side effects.
Now the gossmap_chan_get_capacity() call never fails (we prevented reading
of channels from gossmap in the partially-written case already), so we
make it return the capacity. We do this in msat, because that's what
all the callers want.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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