Make more rxd fields optional. If user has no expiry, consider it active

To accomodate the Montréal section.

• Do not declare an error if the data received from the central server is missing data that is really optional, like membership number or imis_id.
• During expiry processing, if a user has no expiry date, consider it active rather than expired.
• In the log, print the section being processed. This helps in case the same plugin is used to import members from multiple sections.