lnwallet/channel: move common calculation of balance into computeView
lnwallet/channel: move common calculation of balance into computeView

This commit moves common logic used to calculate the state
of a commitment after applying a set of HTLC updates, into
the new method computeView. This method can be used when
calculating the available balance, validating the sanity
of a commitment after applying a set of updates, and also
when creating a new commitment, reducing the duplication
of this logic.

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