Incompatibility with Boost 1.87 due to use of boost::asio::io_service

Hi, just a quick heads-up that biometryd currently doesn't compile with Boost 1.87.

The removed io_service is used in the executor and runtime. My understanding is that it has been replaced with https://www.boost.org/doc/libs/1_87_0/doc/html/boost_asio/reference/io_context.html