Make audit of account group membership changes pluggable

Account group membership changes were logged in a database table.

Now, the audit log service allows to register listeners to group
member added and group member deleted events. A default listener logs
these events to the database as before. But additional listeners may
now be registered for these events using the GroupMemberAuditListener
interface.

Change-Id: I73872c26479a78be31bacc2cbc8eb850a097d473