Prework

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Proposal

1. Record the host and PID of the callr process that runs the pipeline. Store that info in _targets/meta/process .
2. Support a new tar_process() function to retrieve this info. For Compatibility with Shiny #291, let's also throw in tar_pid() just to get the PID.
3. If a new process tries to run the pipeline and the old process is still alive, throw an informative error recommending to terminate the original pipeline or wait until it finishes.

(3) is not perfect, but it is good enough for #291. Following @mattwarkentin's idea from #273, we might even add a little note in tar_watch() because we can be honest that we may not necessarily be able to check if the main process is alive.