when multiple images are requested for download all the temporal activities fail due to heartbeat timeout

[Jacopo Rota]

In 3.5.0 when you donwload multiple images at the same time the network is saturated and the activities are killed due to the heartbeat timeout set to 10 seconds. In particular

```
REPORT_INTERVAL = timedelta(seconds=10)
HEARTBEAT_TIMEOUT = timedelta(seconds=10)
DISK_TIMEOUT = timedelta(minutes=15)
DOWNLOAD_TIMEOUT = timedelta(hours=2)
MAX_SOURCES = 5
CHUNK_SIZE = 5 * (2**20) # 5 MB

            async with self.session.get(
                url, verify_ssl=False, chunked=True
            ) as response, lfile.astore(autocommit=False) as store:
                response.raise_for_status()
                last_update = datetime.now()
                async for data in response.content.iter_chunked(CHUNK_SIZE):
                    activity.heartbeat()
```

we assume to be able to download 5MB of data every 10 seconds, which is not correct in environments with slow network connectivity or when multiple images are downloaded at the same time and the network is saturated.

As a result, the images are never downloaded