read only: don't send active support notifications

Fixes not sending active support notifications when `read_only` is set to to `true`.

`read_only` is overridden for `snappy_store`, although it may be the wrong place to do it—this is because IdentityCache has it's own way of handling a `read_only` state. So when we want to go `read_only`, we'll set it on `Rails.cache` as well as on IDC directly.

@fbogsany @csfrancis @camilo @arthurnn 
Nifty - I haven't seen this before. I prefer `assert @cache.read_multi("walrus", "narwhal").empty?`, but I guess this gives us a nicer error message for free.