Create an explicit EventLoop class which can be used to run an event loop

Create an explicit EventLoop class which can be used to run an event loop
outside of main.cc - i.e. for test cases. Refactor the event module into two
sections - EventScheduler which schedules events, and EventDispatcher which
dispatches events unconditionally. Refactor the signal handling code in
main to use an event dispatcher too, making the loop cleaner.