make :feature annotations also apply a minimum cyrus version requirement

[elliefm]

We have a bunch of tests that are annotated with things like :Conversations to enable configuration to support testing that feature. These tests end up also needing to be annotated with something like :min_version_3_0 due to the feature in question not existing prior to then.

It'd be nice to have a way to associate feature annotations with versions, so that a :Conversations annotation is enough to both enable the feature for the test, and skip the test if the feature doesn't exist yet.

(There's a bunch of such annotations, I chose :Conversations here as a convenient example.)