Agent sidecar may write uncompleted backup file to object storage

[rinx]

Describe the issue:

When the backup files become large, sometimes agent sidecar writes uncompleted backup files to object storage bucket.
This is because agent NGT does not write backup files at once.

Solution:

agent NGT writes backup metadata file after finished to write whole of the backup files.
agent sidecar watches metadata changes.

Environment:

• Go Version: 1.14.3
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.11.5

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[rinx]

resolved in #556