[3407] Added IOSignal class to send signals to IOService
Created d2::IOSignal and d2::IOSignalQueue for propagating caught OS signals to IOService instances. Added TimedSignal test class. New files: src/bin/d2/io_service_signal.cc src/bin/d2/io_service_signal.h src/bin/d2/tests/io_service_signal_unittests.cc