Support multi-threads or multi-processes

[BECATRUE]

Describe the bug

I mistakenly thought Django always operates on a single-thread😅
It might work on multi-threads or multi-processes, hence we should handle it.

Correction methods

As I found, in default it works on a single-thread and single-process.
Nevertheless, it looks necessary to operate the APIs safely, hence I will force several APIs need a common lock.
If we run the server on multi-processes, we need a special lock supporting multi-processes like redis, however, now it looks enough to use just a threading.