Make ReferenceWithHistory output easier to use

Make ReferenceWithHistory easier to use by outputting a timestamp for
each set() operation, and providing a stable "set id" for historic
entries. This means consecutive dump() calls will assign the same ID to
the same historic entry, enabling easier comparison.

This doesn't change the contract of the storage, just the dump() output.

Test: treehugger
Bug: 149014708
Change-Id: I28919a25b09b70910873c40562b51fa6664103b0