consider whether to use @inlineCallbacks (in tests or more generally)

[daira]

<daira> meejah: we don't use inlineCallbacks in Tahoe mainly due to inertia -- the vast majority of the codebase was written without them... <daira> I had some minor technical reservations about them (and about mixing the two styles) but I should probably reconsider whether those are really justified <daira> currently our only use of them is in the tests for some of the cloud backend service providers (on a dev branch) that Leif wrote <daira> I think there's no objection to using them more generally in tests

For this ticket, understand pros and cons of using @inlineCallbacks and add a section to the CodingStandards with guidelines for their use. (This might be very short if no guidelines are needed!)