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 
`handle_expired_entry` also calls `write_entry` (and `delete_entry`): https://github.com/rails/rails/blob/master/activesupport/lib/active_support/cache.rb#L562-L576. Do we want to allow that?