lnwallet: add new restoreHtlc method to the updateLog
lnwallet: add new restoreHtlc method to the updateLog

This commit adds a new method to the updateLog which will be used when
restoring the state of a channel from disk after a restart. This new
method will add an entry to the updateLog without incrementing either
of the counters as the HTLC already comes pre populated with its
historical index.

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