Engineering a Reactor
 -- how to write a non-blocking network library from scratch.

Step 0: a do-nothing event loop
  EventLoop, assertInLoopThread()
  test1: poll for 10 seconds
  test2: abort() as it accesses event loop created in a different thread

Step 1: prototype of reactor
  Channel, Poller, updateChannel().
  test1: poll for ever
  test2: same as before
  test3: poll a timerfd, which expires in 5 seconds

Step 2: TimerQueue
  TimerQueue, Timer, TimerId, EventLoop::runAfter, EventLoop::runEvery
  test3: use EventLoop::runAfter

Step 3: wakeup(), runInLoop()

Step 4: Acceptor

Step 5: TcpServer

Step 6: TcpConnection up and down

Step 7: TcpConnection read

Step 8: TcpConnection write

Step 9: multithreaded TcpServer

Step 10: Connector

Step 11: TcpClient

Step 12: epoll