Fix issues around dicom_ul::get_client_pdu

[Enet4]

Summary

• [ul] adjust get_client_pdu to prevent trailing data loss
  • the function now receives the reader buffer from the caller, thus being able to retain trailing bytes after the returned PDU
  • leave a note that association establishment is still buggy because it does not save the buffer used for receiving the association response (Stream loss in association establishment when association acceptor sends more PDUs in quick succession #589)
• [ul] optimize get_client_pdu to not allocate a Vec per reader buffer fill
• [scpproxy] Tweak dicom-scpproxy to handle `dicom_ul closed connection errors correctly