FIX: Don't use DistributedCache to store redis readonly state

This can cause unbound CPU usage in some cases, and excessive logging in other cases. This commit moves redis readonly information into the local process, but maintains the DistributedCache for postgres readonly state.

spec/requests/forums_controller_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe ForumsController do
     end
 
     it "returns a readonly header if the site is read only" do
-      Discourse.received_readonly!
+      Discourse.received_postgres_readonly!
       get "/srv/status"
       expect(response.status).to eq(200)
       expect(response.headers['Discourse-Readonly']).to eq('true')