public int getThreadAlive() {
        return threadAlive.get();
    }
Thread pool for workers.<br><br>
Use {@link java.util.concurrent.ExecutorService} as inner implement. <br><br>
New feature: <br><br>
1. Block when thread pool is full to avoid poll many urls without process. <br><br>
2. Count of thread alive for monitor.

@author code4crafer@gmail.com
@since 0.5.0
webmagic-core/src/main/java/us/codecraft/webmagic/thread/CountableThreadPool.java