[PATCH] maxFrequency for Ajax.PeriodicalUpdater's decay

Here's a patch for this. I seem to want this in every single project where I use PeriodicalUpdater.

I decided to go with maxFrequency because it's more idiomatic for the user ("wait no more than maxFrequency seconds between requests" vs. "wait no more than frequency divided by maxDecay seconds between requests"). The most efficient implementation of this requires some refactoring; I changed PeriodicalUpdater so that it keeps track of the accumulated decay by changing this.frequency instead of this.decay.

This patch also rolls in the change from #9321, since decaying is broken without it. If you commit that first, I'll be happy to update the diff for this patch against the fixed trunk. For now, it's patched against the broken trunk. So if you commit this first, forget #9321.

updated for trunk as of [7353] as promised

The "frequency" term is really misleading as it really is a period (in seconds), not a frequenci (in Hz). The result which we want to achieve can be termed as either a "minimum frequency" or "a maximum period".

I have chosen the option "frequencyMin" for this purpose. It's really a "periodMax" but at least it's coherent with the whole inversion thing.

Here is a complete and similar patch against trunk src (periodical-updater-frequency-min.patch), it is trivila and vitally useful for many applications. Think about a chat, where you can use decay to reduce load and give faster updates to posters. But you need to guarantee a minimum refresh rate.

Please consider for next release, it's a backward compatible patch.