routing: use updated sphinx.NewOnionPacket method
routing: use updated sphinx.NewOnionPacket method

In this commit, we update the routing package to use the new
`sphinx.NewOnionPacket` method. The new version of this method allows us
to specify _how_ the packet should be filled before it's used to create
a mix-header. This isn't a fundamental change (totally backwards
compatible), instead it plugs a privacy leak that may have revealed to
the destination how long the true route was.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd