lnd: perform HTLC forwarding to switch in distinct goroutine
lnd: perform HTLC forwarding to switch in distinct goroutine

This commit optimizes the previous deadlock bug-fix within the peer’s
channelManager which handles driving the LCP state machine with
additional context-specific state.

Rather than forwarding to the HTLC switch within the primary loop which
handles fully locked-in HTLCs, we now launch a distinct goroutine which
is responsible for properly forwarding lock-in HTLC’s to the
htlcSwitch.

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