Defaulting to only looking at last 90 days of activity, can be adjusted up or down.
$config['tag']['cutoff'] = 86400 * 90;
$config['popular']['cutoff'] = 86400 * 90;
Per-user and per-group tag clouds do not use the cutoff (and it doesn't help with indexing on them).
Consolidated several separate implementations of the same weighting algorithm into common_sql_weight() and fixed some bugs...
For MySQL, now using timestampdiff() instead of subtraction for the comparison, so we get sane results when the year doesn't match, and utc_timestamp() rather than now() so we don't get negative ages for recent items with local server timezone.
Unknown whether the same problems affect PostgreSQL, but note that it lacks the timestampdiff() SQL function.
