multi: thread bandwidth check amount down to MayAddOutgoingHtlc

Pass htlc amount down to the channel so that we don't need to rely
on minHtlc (and pad it when the channel sets a 0 min htlc). Update
test to just check some sane values since we're no longer relying
on minHtlc amount at all.

routing/pathfind.go

@@ -373,7 +373,9 @@ func getOutgoingBalance(node route.Vertex, outgoingChans map[uint64]struct{},
 			}
 		}
 
-		bandwidth, ok := bandwidthHints.availableChanBandwidth(chanID)
+		bandwidth, ok := bandwidthHints.availableChanBandwidth(
+			chanID, 0,
+		)
 
 		// If the bandwidth is not available, use the channel capacity.
 		// This can happen when a channel is added to the graph after