cache-sync policy

This allows to specify when uWSGI should fetch cache dump from remote nodes, we can set it to only fetch it if remote cache has bigger last_modifed_at timestamp or contains more items, or to always fetch (default).

The only downside is that if cache-sync rejects remote dump then we have
```uwsgi_response_write_body_do(): Connection reset by peer [core/writer.c line 205]```
on remote node, I don't know how to close the connection so that other side won't consider it an error. I don't want  to read whole cache dump response since it might be very big.

please review