lnwallet: add new modified HTLC index to the updateLog
lnwallet: add new modified HTLC index to the updateLog

In this commit, we add a new index to the HTLC log. This new index is
meant to ensure that we don't attempt to modify and HTLC twice. An HTLC
modification is either a fail or a settle. This is the first in a series
of commits to fix an existing bug in the state machine that can cause a
panic if a remote node attempts to settle an HTLC twice.

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