Allow multiple assets to use 2wp with different fedpeg scripts

Doing the 2wp more generic would allow to have multiple pegs in the same chain.
Initially it would be fine if all the pegged assets need to be genesis assets (see https://github.com/ElementsProject/elements/pull/164 ), but ideally issued assets should be usable for 2wp too.

In a future step, we can make the 2wp even more generic and allow pegs from different part chains (although maybe it makes sense to do both at the same time).

This interferes with https://github.com/ElementsProject/elements/issues/118