Throttle Rx.Observable without skipping values
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`Throttle` method skips values from an observable sequence if others follow too quickly. But I need a method to just delay them. That is, I need to **set a minimum delay between items, without skipping any**.

Practical example: there's a web service which can accept requests no faster than once a second; there's a user who can add requests, single or in batches. Without Rx, I'll create a list and a timer. When users adds requests, I'll add them to the list. In the timer event, I'll check wether the list is empty. If it is not, I'll send a request and remove the corresponding item. With locks and all that stuff. Now, with Rx, I can create `Subject`, add items when users adds requests. But I need a way to make sure the web service is not flooded by applying delays.

I'm new to Rx, so maybe I'm missing something obvious.