routing+routerrpc: take max cltv limit into account within path finding

With the introduction of the max CLTV limit parameter, nodes are able to
reject HTLCs that exceed it. This should also be applied to path
finding, otherwise HTLCs crafted by the same node that exceed it never
left the switch. This wasn't a big deal since the previous max CLTV
limit was ~5000 blocks. Once it was lowered to 1008, the issue became
more apparent. Therefore, all of our path finding attempts now have a
restriction of said limit in in order to properly carry out HTLCs to the
network.

lnrpc/routerrpc/router.pb.go

@@ -212,8 +212,9 @@ type SendPaymentRequest struct {
 	//any channel may be used.
 	OutgoingChanId uint64 `protobuf:"varint,8,opt,name=outgoing_chan_id,json=outgoingChanId,proto3" json:"outgoing_chan_id,omitempty"`
 	//*
-	//An optional maximum total time lock for the route. If zero, there is no
-	//maximum enforced.
+	//An optional maximum total time lock for the route. This should not exceed
+	//lnd's `--max-cltv-expiry` setting. If zero, then the value of
+	//`--max-cltv-expiry` is enforced.
 	CltvLimit int32 `protobuf:"varint,9,opt,name=cltv_limit,json=cltvLimit,proto3" json:"cltv_limit,omitempty"`
 	//*
 	//Optional route hints to reach the destination through private channels.