Send update_channel messages to re-enable a disabled channel

Send update_channel messages to re-enable a disabled channel

Currently, we only send an update_channel message after
disconnecting a peer and waiting some time. We do not send a
followup when the peer has been reconnected for some time.

This changes that behavior to make the disconnect and reconnect
channel updates symmetric, and also simplifies the state machine
somewhat to make it more clear.

Finally, it serializes the current announcement state so that we
usually know when we need to send a new update_channel.