Socket: Ignore ECONNREFUSED on write to datagram socket

Examples/RateStuffer: Use CloneSource module
Exmaples/RateStuffer: Implement using a RateStuffer subnet
PPI: Implement CloneSource
PPI: Add throttle()/unthrottle() to passive debug modules
PPI: Implement DiscardSink
PPI: Implement ThrottleBarrier
PPI: Implement missing unit tests
PPI: Complete the documentation
Scheduler: Improve ClockService performance and remove a race condition
Scheduler: Clean up Scheduler::process()
Utils: Make singleton::instance() protected by default and fix implementation
Remove private members from doxygen documentation