This uses the same state machine as HTLCs, but they're only
ever added, not removed. Since we can only have one in each
state, we use a simple array; mostly NULL.
We could make this more space-efficient by folding everything into the
first 5 states, but that would be more complex than just using the
identical state machine.
One subtlety: we don't send uncommitted fee_states over the wire.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
