breacharbiter: receive breaches to ACK on separate channel
breacharbiter: receive breaches to ACK on separate channel

This commit changes how the breachArbiter gets notified about channel
breaches. Previously it would need to SubscribeToChannelEvents to get
get notified if any breach happened, now we send all seen breaches on a
new channel ContractBreaches.

By having the breachArbiter subscribe to channel events, we risked
events getting lost when we were either starting up or shutting down,
since events could happen before we had been able to subscribe, or right
after we had cancelled our subscription.

Now it is the server's responsibility to reliably forward events from
the ChainArbitrator to the breachArbiter, and forward the ACK the
breachArbiter responds with. This makes sure that the messages aren't
lost in the event of starting up or shutting down, since the connection
between the subsystems now are static.

A result of this change is that the internals of the breachArbiter can
be simplified significantly, as we will get all channel breaches
forwarded on one channel. This lets us get rid of the observer
goroutines, and we spin up goroutines handling the channel breaches only
when they happen.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd