Fix build with Boost >= 1.66.0
It should eventually be changed from io_service to io_context (see https://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/net_ts.html) but that's for a different day.
It should eventually be changed from io_service to io_context (see https://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/net_ts.html) but that's for a different day.