Port to Boost.Process V2

Boost.Process now has a major V2 rewrite: https://www.boost.org/doc/libs/1_87_0/doc/html/boost_process/v2.html

Porting is likely to be nontrivial due to Boost.Asio dependencies, but it may be worth leaning into Boost.Asio anyway to support #6, #15, etc.

Since most RLS processing is timer-based, it may be possible to let a Boost.Asio `io_context` take over the main thread.