refactor main function

[chai2010]

1. recall main to restart metad is unsafe when panic, fix it
2. simplify pprof http server goroutine start
3. log.Fatal will exit, the os.Exit is unused

[martinyunify]

I'm wondering if it is appropriate to change the default behavior although the behavior is not safe. I think maybe we can utilize golang safe restart library such as https://github.com/fvbock/endless/

[chai2010]

IMO, any server should exit when panic.
the endless is only for zero downtime restarts servers, not restart for panic.
and the endless solution is too complex.

it we need auto restart metad, we should use systemd.

[martinyunify]

yes. I think some descriptions should be added when default behavior is changed. Either providing systemd definition or descriptions in documents would be fine.

[chai2010]

add the docs/service.md to describe how to run metad as a service, and auto restart.