routing: limit routing info size during pathfinding

Also the max hop count check can be removed, because the real bound is
the payload size. By moving the check inside the search loop, we now
also backtrack when we hit the limit.

routing/payment_lifecycle.go

@@ -196,7 +196,6 @@ func errorToPaymentFailure(err error) channeldb.FailureReason {
 		errNoTlvPayload,
 		errNoPaymentAddr,
 		errNoPathFound,
-		errMaxHopsExceeded,
 		errPrebuiltRouteTried:
 
 		return channeldb.FailureReasonNoRoute