* Add periodic callhome functionality
Periodically (every 24hrs by default), fetch callhome information and
upload it to SUBNET.
New config keys under the `callhome` subsystem:
enable - Set to `on` for enabling callhome. Default `off`
frequency - Interval between callhome cycles. Default `24h`
* Improvements based on review comments
- Update `enableCallhome` safely
- Rename pctx to ctx
- Block during execution of callhome
- Store parsed proxy URL in global subnet config
- Store callhome URL(s) in constants
- Use existing global transport
- Pass auth token to subnetPostReq
- Use `config.EnableOn` instead of `"on"`
* Use atomic package instead of lock
* Use uber atomic package
* Use `Cancel` instead of `cancel`
Co-authored-by: Harshavardhana <harsha@minio.io>
Co-authored-by: Harshavardhana <harsha@minio.io>
Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>
