Undetected issue payment

[sander2]

As reported by Quarkslabs:

Description: If a user makes multiple issue requests and pays them in the same Bitcoin trans-action, then the second payment output will be missed by the vaults. This is due to the use of find_map in the process_transaction_and_execute_issue function which only returns the first matching element as stated in its documentation.
Recommandation: A possible fix for this issue would be turning the if let construct into a while let construct. However this alone should not be enough as the core of the function can return and needs to be changed accordingly.