add an example to demonstrate how to leverage sockets concurrently in multithreading scenarios

[luxiaoshuang@…]

The ideal solution of leveraging sockets concurrently in multithreading scenarios should 1. not block io workers (the threads where io_service.run() is invoked), 2. be free of lock race, 3. etc.

There are many threads when you are searching this issue in google, but usable code is hard to find.

In this proposal, I offer a reference implementation to the people who wanna call async_write family in multiple threads.

[luxiaoshuang@…]

patch