This introduces a new TimeoutSet class for use in
EventLoopImplementationUnix. It is responsible for finding a timer that
expires the soonest and for firing expired timers. TimeoutSet expects
timeouts to be subclasses of EventLoopTimeout, of which EventLoopTimer
is now a subclass, obviously.
TimeoutSet stores timeouts in a binary heap, so
EventLoopImplementationUnix should handle large amounts of timers a lot
better now.
TimeoutSet also supports scheduling of timeouts whose fire time is
relative to the start of the next event loop iteration (i. e. ones
that directly bound polling time). This functionality will reveal its
full potential later with the implementation of asynchronous sockets but
it is currently used to implement zero-timeout timers that are an analog
of Core::deferred_invoke with slightly different semantics.
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