[boost] Bug in Boost.Asio strand(strand<OtherExecutor>&& other)

Hello, there is a bug in the Boost.Asio strand::strand(strand<OtherExecutor>&& other) move constructor as it tries to do an invalid cast on the wrong object. A friend declaration is also required to access the private members of the other strand. I submitted a PR https://github.com/boostorg/asio/pull/333 to address this issue. Regards, Niklas