fix: give each worker process a unique locker identity

[pplulee]

Summary

• Postal.locker_name was host:<hostname> pid:<pid> thread:<id>. Docker workers often share hostname and PID, so every process renewed the same worker_roles row and ran scheduled tasks concurrently.
• Add a per-process random id to process_name so each worker has a distinct locker identity.

Fixes #3627

Test plan

• Run two or more worker containers with the same hostname.
• Confirm worker_roles.worker contains different id: values after restart.
• Confirm only one worker logs running task / checking DNS for CheckAllDNSScheduledTask at :15.

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