* use popen to run updater/analytics in detached mode
subprocess does not provide a way to avoid a zombie process. Not to confuse with orphaned processes, which will be `wait`-ed by `pid 1`.
It might not work for long-running processes, but I don't think zombie processes are going to be a problem for us, as the parent process immediately exits after spawning a child process.
Even in case of `queue` based experiment runner, parent will exit before than the child process. So I don't think it's a requirement here. But it does feel wrong to leak the processes, so I have to sleep on that.
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This PR:
1. makes `daemonized` process inherit the logging verbosity, so if the parent process used `-vv`, the daemonized process will run the same way.
2. Add `DVC_DAEMON_LOGFILE` envvar, that the daemon will log to the file. By default, the verbosity is the same as running parent processes. Without `DVC_DAEMON_LOGFILE`, the output will be redirected to `/dev/null`.
3. Adds debug/trace for updater/analytics.
I hope by doing this, we can make our life easier on debugging and make daemon more transparent/observable.
* add tests
* fixes
* use kwargs
* pass all os.environ with some modified
skshetry