Fix for short jobs run in AthenaMP with SharedWriter

This MR fixes two issues related to very short AthenaMP jobs (where some worker may not process any events) using SharedWriter:

• If a worker finishes and sends metadata, before other workers send their first event data, it may cause an overwrite of the streamPortString. To make this save, use a dedicated field for each worker.
• Fix logic on when to finish server loop

This MR only affects AthenaMP with SharedWriter, e.g. Derivation not serial or multithreaded Athena, nor bare AthenaMP (with separate merge). It should be safe to be accepted, but can wait if needed (hopefully not for weeks though!).