lnwallet/channel: populate htlc indexes from disk commit
lnwallet/channel: populate htlc indexes from disk commit

We currently write each HTLCs OutputIndex to disk, but we don't use it
when restoring. The restoration is modified to use these directly, since
we will have lost access to the sorting of CLTVs after the initial
signing process.

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