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 
We override `read_multi`, so we don't care about that one.

I don't think we should allow any modification of memcached when `read_only` is true. If an entry has expired, the clients will simply ignore it & return `nil`.