Use std::atomic for service request ID counter

[christophebedard]

https://en.cppreference.com/w/cpp/atomic/atomic

It's also what's used in rmw_cyclonedds: https://github.com/ros2/rmw_cyclonedds/blob/6cec3393d1ac695eb11f42c1f324b1832c11ff86/rmw_cyclonedds_cpp/src/rmw_node.cpp#L3697