Run permanent sync thread after app.run ()

[LeonidVas]

When the service runned by gunicorn, the code runs in the "workers", which are "pinged" by gunicorn. If the "worker" is silent for more than the value of the timeout option, the worker will be killed and restarted.
The "worker" is currently created when the service starts and can't be pinged by gunicorn before the app.run () was called.
The problem is that all repositories are synced when the service starts (before app.run ()) because:

1. this sync can take a long time
2. it must be executed once (in one thread). So. we can't run gunicorn with workers > 1.

Both problems can be solved by starting the server by gunicorn with the preload option, but in this case we should to run
permanent sync thread after calling app.run (). Now it's run when s3model is created.

[LeonidVas]

May be useful https://www.py4u.net/discuss/141357