cleanup code is incomplete

[bstarynk]

In commit 8ec3726 we do have some

--cleanup

program option. That program flag is supposed to be run by ./helpcovid launched from some crontab(5) job running e.g. every five or ten minutes. It runs the hcv_background_periodic_cleanup C++ function in C++ file hcv_background.cc which should also be run periodically.

That hcv_background_periodic_cleanup C++ function is incomplete, and should cleanup both the database and any global data structure (protected by some std::mutex or std::recursive_mutex).
We may need to use also some std::condition_variable and/or synchronize threads using hcv_bg_event_fd so eventfd(2).

We might also synchronize threads using pipe(7) or temporary fifo(7).