Be More Conservative on Sending Shutdown

[TheBlueMatt]

cc lightning/bolts#964 but we need to update our shutdown sending/handling - we should accept a shutdown even if there's pending updates, and we also need to consider update_fail/update_fail_malformed and update_fee as "pending updates".

[dunxen]

If this is still relevant I'd be happy to run with it, unless there is some other deeper context I'd need to be aware of / cannot easily discover in LDK's handling.

[TheBlueMatt]

I'd let this one sit for a bit. There continues to be some debate in the BOLTs as to the correct direction here, with multiple alternative PRs going different directions.

[TheBlueMatt]

May have been at least partially addressed by #2529?