httpserver: explicitly detach worker threads

When using std::thread in place of boost::thread, letting the threads destruct
results in a std::terminate. According to the docs, the same thing should be
be happening in later boost versions:
http://www.boost.org/doc/libs/1_55_0/doc/html/thread/thread_management.html#thread.thread_management.thread.destructor

I'm unsure why this hasn't blown up already, but explicitly detaching can't
hurt.