make tag dropoff configurable

darcs-hash:20080811175820-84dde-f3d934495fa90fadde5f7d5d0c37c5f2a575a9d8.gz

config.php.sample

@@ -71,4 +71,8 @@ $config['db']['database'] = 'mysql://laconica:microblog@localhost/laconica';
 #Do notice broadcasts offline
 #If you set this, you must run the xmppdaemon.php
 #as a daemon (with, say, start-stop-daemon)
-#$config['queue']['enabled'] = true;
+#$config['queue']['enabled'] = true;
+
+#exponential decay factor for tags, default 10 days
+#raise this if traffic is slow, lower it if it's fast
+#$config['tag']['dropoff'] = 86400.0 * 10;