BOLT 2: upgrade protocol on reestablish

[rustyrussell]

This is the simplest upgrade mechanism I could come up with. It's ready for option_anchors_zero_fee_htlc_tx, too.

Note the reason it's on reconnect: whatever we do, we need to handle reconnect during an upgrade attempt, which meant some kind of fallback "where were we up to?" at that point. Simplest to make that "fallback" technique the only technique.

And in practice we don't upgrade software without reconnecting anyway.

[rustyrussell]

Rebased and further simplified.

[TheBlueMatt]

Concept ACK, can we drop it being based on quiescence? Seems like we can parallelize those efforts.

[rustyrussell]

Rebased, and added an explicit feature bit. As a bonus I threw in a commit which defined what "negotiated" and "offered" mean for features!