This could always happen, but now we're more correct about send it, it's
a possible race that I've seen in CI.

We can never get a PONG (that's now handled by connectd directly), but we
can definitely see this:

```
2025-05-15T02:29:14.7403081Z         # Even with onchain anchor channel, it still keeps reserve (just in case!).
2025-05-15T02:29:14.7403766Z >       l1.rpc.close(l2.info['id'])
2025-05-15T02:29:14.7404069Z 
2025-05-15T02:29:14.7404248Z tests/test_opening.py:2536:
...
2025-05-15T02:29:14.7413090Z >       return self.sock.recv(length)
2025-05-15T02:29:14.7413353Z E       Failed: Timeout >1800.0s
...
2025-05-15T02:29:14.8097229Z lightningd-1 2025-05-15T01:50:59.538Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-closingd-chan#1: billboard perm: Expected closing_signed: 0103fad81b49d5e367cd785e3088b9acf356a7984a17a1ccda86d09da41438840b08000067000001000078c3314666731e339c0b8434f7824797a084ed7ca3655991a672da068e2c44cb53b57b53a296c133bc879109a8931dc31e6913a4bda3d58559b99b95663e6d52679cabaccc6e39dbd95a4dac90e75a258893c3aa3f733d1b8890174d5ddea8003cadffe557773c54d2c07ca1d535c4bf85885f879ae466c16a516e8ffcfec174
```

Signed-off-by: Rusty Russell <[email protected]>

In this case a "are we closed onchain".

Signed-off-by: Rusty Russell <[email protected]>

We want a more fine-grained approach, so we now have:

1. update_channel_update() - returns true if it changed.
2. channel_should_enable() - should this channel be marked enabled.
3. arm_refresh_timer() - start a refresh timer for the channel_update.

Signed-off-by: Rusty Russell <[email protected]>

This message was too verbose (even for trace!)

Signed-off-by: Rusty Russell <[email protected]>

They can all call get_block_height(); the extra argument confused me and
I thought they were called before the block height was actually updated.

Signed-off-by: Rusty Russell <[email protected]>

… wait until 6 deep.

The spec used to say you had to wait for channel to be ready, *and* 6
depth before exchanging signatures.  Now the 6 depth requirement is only
on the actual announcing of the channel: you can send sigs any time.

This means our state machine goes from:

  NOT_USABLE -> NOT_DEEP_ENOUGH -> NEED_PEER_SIGS -> ANNOUNCED

to:

  NOT_USABLE -> NEED_PEER_SIGS -> NOT_DEEP_ENOUGH -> ANNOUNCED

However, this revealed that our state machine is insufficient, so
rework it to be more general and understandable.  In particular,
check for unexpected state transitions, and thus document them.

Note that cg->sent_sigs replaces channel->replied_to_announcement_sigs,
too.

Signed-off-by: Rusty Russell <[email protected]>
Changelog-Changed: Protocol: We now exchange `announcement_signatures` as soon as we're ready, rather than waiting for 6 blocks (as per recent BOLT update)