sr: Check why some sr_disk_state fields need to be cleared

teor: When we add fields, how will we make sure we remember to clear them all?

nickm: We don't need to do so in general, the manager will take care of it for us. This is only necessary for derived fields, and fields for which the unit tests are expecting an unusual behavior, IIRC.

nickm: (I can dig deeper here; I forget the exact reason why this function is clearing these, but I think it is because of legacy cruft.)

teor: Yeah let's double-check what's going on here.

https://github.com/nmathewson/tor/pull/3#discussion_r317548828

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