Stop losing channels when nodes are modified.

[baptr]

SetNode was deleting the old copy of the node before writing the new one, which cascaded to delete channels.

Now the new node is set up in place, which maintains more state. If it's a rename or delete, the old node is cleaned up after.

This probably intersects with https://github.com/google/shenzhen-go/commit/717891c3b9f6b0870c841e34cbf9b40a27f2b878, but opening it for discussion anyway.