Revert "Revert "We no longer ever talk to the same boost::asio::ip::udp::socket from more than one thread at once.""

This reverts commit aba6432cfcb4c0431a34a2e667c62931e075013f.
Turns out the race condition fix only deadlocked under debug builds due to assertion code :). Note to self: test non-optimised builds in future!