Created by: wangkuiyi

• Move implementation details from threadpool.h into threadpool.cc, so that users can understand the design with a single glance.
• Data members, including static ones, should follow the naming convention of data_member_.
• Add the missing #include <vector>.
• Remove ThreadPool::GetAvaialble and ThreadPool::GetNumThreads because they are not used and hopefully will not be used.
• Rename ThreadPool::num_threads_ into ThreadPool::total_threads and ThreadPool::available_ into ThreadPool::idle_threads.