peer: fix last-mile settle stalling in concurrent multi-hop setting
peer: fix last-mile settle stalling in concurrent multi-hop setting

This commit fixes an issue that would at times cause the htlcManager
which manages the link that’s the final hop to settle in an HTLC flow.
Previously, a case would arise wherein a set of HTLC’s were settled to,
but not properly committed to in the commitment transaction of the
remote node. This wasn’t an issue with HTLC’s which were added but
uncleared, as that batch was tracked independently.

In order to fix this issue, we now track pending HTLC settles
independently. This is a temporary fix, as has been noted in a TODO
within this commit.

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