Fix build with boost 1.87

[antonio-rojas]

io_service is gone, port to io_context

[JohnCremona]

io_service is gone, port to io_context

Thanks! I was worried that this would break for people without the latest boost, but I see that in my own /usr/include/boost/asio/io_service.hpp there is the line
typedef io_context io_service;
so I expect that will be OK?

[antonio-rojas]

It works at least as far back as 1.81