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/unified_policies.go

@@ -170,7 +170,7 @@ func (u *unifiedPolicy) getPolicyLocal(amt lnwire.MilliSatoshi,
 		// channel. The bandwidth hint is expected to be
 		// available.
 		bandwidth, ok := bandwidthHints.availableChanBandwidth(
-			edge.policy.ChannelID,
+			edge.policy.ChannelID, amt,
 		)
 		if !ok {
 			bandwidth = lnwire.MaxMilliSatoshi