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 
`delete` returns the return value of `delete_entry`, but you're overriding `delete` so that doesn't matter.

`read`, `read_multi`, and `exist?` depend on `read_entry`'s return value, tho you're not overriding that so it doesn't matter.

`write` will return `write_entry`'s return value, but you're overriding `write` so it doesn't matter.

I guess I'd argue for `return true if read_only` purely for interface consistency, but there's a lot of "it doesn't matter" above, so nvm.