lnwallet: fix race condition when forwarding by creating new pubkey
lnwallet: fix race condition when forwarding by creating new pubkey

This commit fixes a race condition that would at times occur in the
htlcswitch.TestChannelLinkBidirectionalOneHopPayments test case. A race
condition would occur in the goroutine running ReceiveNewCommitment
compared with the grouting that would obtain the snapshot in order to
make a forwarding decision.

We fix this by creating a new public key for each new commitment
transaction such that we complete avoid the read/write race condition.

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