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/components/discourse_spec.rb

@@ -219,8 +219,13 @@ describe Discourse do
         expect(Discourse.readonly_mode?).to eq(true)
       end
 
-      it "returns true when Discourse is recently read only" do
-        Discourse.received_readonly!
+      it "returns true when postgres is recently read only" do
+        Discourse.received_postgres_readonly!
+        expect(Discourse.readonly_mode?).to eq(true)
+      end
+
+      it "returns true when redis is recently read only" do
+        Discourse.received_redis_readonly!
         expect(Discourse.readonly_mode?).to eq(true)
       end
 
@@ -234,21 +239,28 @@ describe Discourse do
       end
     end
 
-    describe ".received_readonly!" do
+    describe ".received_postgres_readonly!" do
       it "sets the right time" do
-        time = Discourse.received_readonly!
-        expect(Discourse.last_read_only['default']).to eq(time)
+        time = Discourse.received_postgres_readonly!
+        expect(Discourse.postgres_last_read_only['default']).to eq(time)
+      end
+    end
+
+    describe ".received_redis_readonly!" do
+      it "sets the right time" do
+        time = Discourse.received_redis_readonly!
+        expect(Discourse.redis_last_read_only['default']).to eq(time)
       end
     end
 
     describe ".clear_readonly!" do
       it "publishes the right message" do
-        Discourse.received_readonly!
+        Discourse.received_postgres_readonly!
         messages = []
 
         expect do
           messages = MessageBus.track_publish { Discourse.clear_readonly! }
-        end.to change { Discourse.last_read_only['default'] }.to(nil)
+        end.to change { Discourse.postgres_last_read_only['default'] }.to(nil)
 
         expect(messages.any? { |m| m.channel == Site::SITE_JSON_CHANNEL })
           .to eq(true)