storage code review

[graydon]

This is the one change I'm fairly confident ought to happen in storage: factoring the get paths into a common funnel. Besides being duplicate code, if you look carefully the previous arrangement had inconsistent handling of None returns from the underlying map too: one path (correctly) treated it as an internal error since prepare_read_only_access shouldn't allow getting there; the other two paths either returned it as-is or merged it into the MissingValue path.

I'm not changing two other error codes just yet, though I was a bit tempted to. Would be interested to hear input from @dmkozh : as mentioned in conversation elsewhere, I think the footprint checking paths should probably return InvalidAction as errors rather than ExceededLimit; and possibly the path in extend that returns an InternalError live-until value should be InvalidAction too. But I can imagine the counterarguments to these, and if you feel strongly that they're correct already I'm ok leaving them as-is.