The FSM has the right decoding options. First, we change the
`marshallingOptions` field to a slice instead of a single option for
consistency with the remaining of the code (`Serialize()` and
`Parse*()` accepts several options). This is not really needed as we
only initialize a single option or none, but I suppose this may matter
in the future.

Then, we pass the decoding options to the `watchUpdateEvent` struct.
From my understanding, this is the only one needing it. When
transmitting the local RIB, we don't need to encode paths with
AddPath. And when mirroring routes, the routes are already encoded.

I am however unable to correctly pass the decoding options in the
post-policy case. I don't see an obvious way to access the FSM in this
case.