This PR fixes#2312 which are four things:
* getaddressinfo was not implemented in spectrum which results in "method not found"
* There was an unrelated KeyError which occured in Transactions where one the output was a change-address. It's a bit unknown why that hasn't occured earlier. At least it was diffcult to spot as the fetch_transactions call was way to huge and very confusing. So ...
* the fetch_transactions call was refactored in its own class, TxFetcher. As we did that, we also moved two other related files in a newly created wallet-package.
* Tiny bug around tx["amount"] which only exists as tx["flow_amount"] for transactions.
