Fixes performance issue with txproposals

Avoid getting addresses from scripts (that's an expensive computation)
This is a very interesting finding and optimisation!

I am a bit worried of accessing the internal `.vanilla` data structure directly, I double checked and there does not seems to be any security concerns about it (i.e. the peer sending a tricky TXP in which the outs seems to go some address, but there are going to somewhere else). So I am OK with the change. Please consider consider creating a method in `TxProposal`, something like `getRawOutputs`.