Add parallelism to object expirer daemon.

Two types of parallelism are added:

- concurrency to speed up what a single process does
- a way to run multiple daemons to work on different parts of the work

DocImpact

Change-Id: I48997f68eb2fd8de19a5ee8b9fcdf76dde2ba0ab
Merge "Add parallelism to object expirer daemon."