event_loop_thread can't start after fork

[shgxwxl]

We create EventLoopThread and initialize it in father process, but we use it in child process.
If we have only one child process , everything goes well.

But if we have multi child processes, something goes wrong.
Because EventLoop::watcher_ is created and initialized in father process, all children processes inherited father's pipe. When we use the pipe to do a notification in one child process, the notification may be received by another child process randomly.

if it's message had readed by other process
we marked watcher had been notified_ and never notify it again

    if (!notified_.load()) {
        DLOG_TRACE << "call watcher_->Nofity() notified_.store(true)";

        // We must set notified_ to true before calling `watcher_->Nodify()`
        // otherwise there is a change that:
        //  1. We called watcher_- > Nodify() on thread1
        //  2. On thread2 we watched this event, so wakeup the CPU changed to run this EventLoop on thread2 and executed all the pending task
        //  3. Then the CPU changed to run on thread1 and set notified_ to true
        //  4. Then, some thread except thread2 call this QueueInLoop to push a task into the queue, and find notified_ is true, so there is no change to wakeup thread2 to execute this task
        notified_.store(true);

        // Sometimes one thread invoke EventLoop::QueueInLoop(...), but anther
        // thread is invoking EventLoop::Stop() to stop this loop. At this moment
        // this loop maybe is stopping and the watcher_ object maybe has been
        // released already.
        if (watcher_) {
            watcher_->Notify();
        } else {
            DLOG_TRACE << "status=" << StatusToString();
            assert(!IsRunning());
        }
    } else {
         DLOG_TRACE << "No need to call watcher_->Nofity()";
    }

[zieckey]

Fixed 9fc8601