lnwallet: add distinct channel to send close details over
lnwallet: add distinct channel to send close details over

This commit modifies the actions of the closeObserver goroutine to
utilize a _new_ channel to send channel close details over. The
original close signal channel is still used to notify observers that a
channel _has_ been closed, but this new channel will provide a single
observer with details w.r.t _how_ a channel was closed.

aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,aakselrod/lnd,ddustin/lnd,ddustin/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd