Real-time safe services + clients

Make the client/service pipeline safe for real-time execution. It has not yet been vetted for allocations, STL structures which malloc automatically, and nondeterministic blocking.

This will be required for RT-safe node lifecycle, since we plan to use services to transition between states in the lifecycle.

• abstract out "new" in Service::handle_request
• abstract out make_shared from async_send_request
• rt-safe alternatives to std::vector and std::map (maybe just use a custom allocator)
• example with OS-specific threading abstraction (instead of std::future)
• testing/profiling