generated_message_details() -> Simplifications

- avoid using a HashMap for `messages_to_refine`. It seems that a vec is
  enough
- minimize the number of conversions between `OutboundMessageDetails` and
  `MessageDetails`
- use references where possible in order to minimize the number of
  intermediary Vecs
- simplify `make_message_details_map()` logic, reduce its scope and rename
  it to `validate_out_msgs_details()`

Signed-off-by: Serban Iorga <[email protected]>