Download of basebackup always stalls

Hello,

I'm facing the issue where I'm not able to download a basebackup using `pghoard_restore` command since download always stalls.

Restore command:
```
sudo -u postgres pghoard_restore get-basebackup --config pghoard.json --restore-to-master --overwrite --target-dir /var/lib/pgsql/9.5/data-new/
```

The appropriate backup was selected, but nothing happens.
`ps auxf` shows that `pghoard_restore` creates 9 additional processes but the download progress is constantly 0%, which after 3 x 2 minutes turns to fail.

Command output:
```
Found 1 applicable basebackup

Basebackup Backup size Orig size Start time
---------------------------------------- ----------- ----------- --------------------
server-f-postgres-01/basebackup/2019-07-10_12-27_0.00000000.pghoard 13245 MB 35432 MB 2019-07-10T12:27:32Z
metadata: {'compression-algorithm': 'snappy', 'format': 'pghoard-bb-v2', 'original-file-size': '81920', 'host': 'server-f-postgres-01', 'end-time': '2019-07-10 14:33:12.657815+02:00', 'end-wal-segment': '000000010000001A0000004A', 'pg-version': '90518', 'start-wal-segment': '000000010000001A00000048', 'total-size-plain': '37153730560', 'total-size-enc': '13888641735'}

Selecting 'server-f-postgres-01/basebackup/2019-07-10_12-27_0.00000000.pghoard' for restore
2019-07-10 15:20:34,941%BasebackupFetcher MainThread ERROR Download stalled for 120.43377648199385 seconds, aborting downloaders
2019-07-10 15:22:35,674%BasebackupFetcher MainThread ERROR Download stalled for 120.44614975301374 seconds, aborting downloaders
2019-07-10 15:24:36,392%BasebackupFetcher MainThread ERROR Download stalled for 120.47685114300111 seconds, aborting downloaders
2019-07-10 15:24:36,612 BasebackupFetcher MainThread ERROR Download stalled despite retries, aborting
FATAL: RestoreError: Backup download/extraction failed with 1 errors

```

`pghoard.conf`:
```json
{
"backup_location": "./metadata",
"backup_sites": {
"server-f-postgres-01": {
"active_backup_mode": "pg_receivexlog",
"basebackup_mode": "local-tar",
"basebackup_chunks_in_progress": 5,
"basebackup_chunk_size": 2147483648,
"basebackup_hour": 5,
"basebackup_interval_hours": 24,
"basebackup_minute": 40,
"pg_data_directory": "/var/lib/pgsql/9.5/data",
"nodes": [
{
"host": "127.0.0.1",
"user": "postgres",
"password": "secret",
"port": 5432
}
],
"object_storage": {
"storage_type": "google",
"project_id": "postgres-dev",
"bucket_name": "test-pghoard"
}
}
}
}

```