Add date-based expiry to the AWL table via maiadbtool's --prune-awl option

[rjl]

SpamAssassin's AWL table does not have a built-in expiry mechanism, so it will grow without bound. The --prune-awl feature of maiadbtool.pl will expire entries that have fewer than a given number of occurrences, which helps to weed out the one-off entries that take up space, but it would be more effective to add an age-based expiry criterion.

An auto-updating timestamp column can be added to the AWL table, such that timestamps are automatically assigned to new records and existing records have their timestamps updated whenever other columns are changed (e.g. with an "ON UPDATE" clause for MySQL, or a "TRIGGER" for PostgreSQL). This requires no changes to SpamAssassin, as the timestamps are managed entirely by the database.

From there, maiadbtool.pl should only need to look for another (optional) parameter to --prune-awl, specifying the minimum age (in days) of records to delete.

[rjl]

For MySQL:

 ALTER TABLE awl
 ADD COLUMN lastupdate
 TIMESTAMP NOT NULL
 DEFAULT CURRENT_TIMESTAMP
 ON UPDATE CURRENT_TIMESTAMP;

[rjl]

For PostgreSQL:

 ALTER TABLE awl
 ADD COLUMN lastupdate
 TIMESTAMP NOT NULL;

 CREATE FUNCTION awl_stamp() RETURNS trigger AS $awl_stamp$
   BEGIN
     NEW.lastupdate := current_timestamp;
     RETURN NEW;
   END;
 $awl_stamp$ LANGUAGE plpgsql;

 CREATE TRIGGER awl_stamp BEFORE INSERT OR UPDATE ON awl
   FOR EACH ROW EXECUTE PROCEDURE awl_stamp();

[mortonda@…]

I'm uncomfortable making this change so close to a release when it might have a performance impact...