Run forever supports not exit progam

## What is the problem your feature solves, or the need it fulfills?

I use `pingora` as a lib for my reverse proxy , but `run_forever` function will call `std::process::exit(0);`. I expect close the http server only, but not exit the program,.

## Describe the solution you'd like

Support parameter to set not exit the program for `run_forever` function.