peer: eliminate possibility of concurrent map writes in htlcManager
peer: eliminate possibility of concurrent map writes in htlcManager

This commit fixed an issue in the htlcManager goroutine which manages
channel state updates. Due to lack of a mutex protecting the two maps
written in the goroutine launched to forward HTLC’s to the switch.

This issue was detected by golang’s runtime which is able to detect
invalid concurrent map writes.

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