htlcswitch: remove synchronous link handoff, special-case keystone err

This allows Switch-initiated payments to be failed back if they don't
make it into a commitment. Prior to this commit, a Switch-initiated
HTLC could get "lost" meaning the circuit wouldn't get deleted except
if conditions were "right" and the network result store would never
be made aware of the HTLC's fate. Switch-initiated HTLC's are now
passed to the link's mailbox to ensure they can be failed back.

This change also special-cases the ErrDuplicateKeystone error from
OpenCircuits(...) so that callers of updateCommitTx() in the link
don't send an Error to the peer if they encounter the keystone error.
With the first async change, the keystone error should now always
be recoverable.

htlcswitch/switch.go

@@ -484,6 +484,7 @@ func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID, attemptID uint64,
 		incomingHTLCID: attemptID,
 		outgoingChanID: firstHop,
 		htlc:           htlc,
+		amount:         htlc.Amount,
 	}
 
 	// Attempt to fetch the target link before creating a circuit so that
@@ -547,10 +548,11 @@ func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID, attemptID uint64,
 		return ErrLocalAddFailed
 	}
 
-	// Send packet to link.
+	// Give the packet to the link's mailbox so that HTLC's are properly
+	// canceled back if the mailbox timeout elapses.
 	packet.circuit = circuit
 
-	return link.handleLocalAddPacket(packet)
+	return link.handleSwitchPacket(packet)
 }
 
 // UpdateForwardingPolicies sends a message to the switch to update the