sd-event: add ability to ratelimit event sources

sd-event: add ability to ratelimit event sources

Let's a concept of "rate limiting" to event sources: if specific event
sources fire too often in some time interval temporarily take them
offline, and take them back online once the interval passed.

This is a simple scheme of avoiding starvation of event sources if some
event source fires too often.

This introduces the new conceptual states of "offline" and "online" for
event sources: an event source is "online" only when enabled *and* not
ratelimited, and offline in all other cases. An event source that is
online hence has its fds registered in the epoll, its signals in the
signalfd and so on.