``Process`` or ``Subprocess`` to spawn offline ingestor process.

[YooSunYoung]

We can either use multiprocessing or subprocess to start an offline ingestor in the different process.

-	multiprocessing	subprocess
pros	- easy to attach callback - easy to post-process the result of the offline-ingestor - doesn't need to use arbitrary command	- offline-ingestor will need a command entry point anyways, so don't need to implement another accessible interface
cons	- needs a function type interface to run an offline-ingestor	- limited access to the result - not possible to attach a callback function - needs to wrap the pipe-object with an async call to continue to the next message while offline-ingestor is running

Since we don't use non-python program at all,
I would like to go for the multiprocess.Process...?

In case we keep using subprocess, we need to make sure if it should be asynchronously run or not.