Unify FeatureSupport / ServerSupport and other feature support detection functions

Currently, our new threads work ended up introducing multiple solutions for detecting what features the server we're talking to can support. We've also got yet another bit of code to determine what features our client supports.

Ideally, we'd unify the API for all of these (Threads.hasServerSideSupport, client.canSupport.get(Feature.…), client.supportsExperimentalThreads(), etc).