FEATURE: improve performance of anonymous cache

This commit introduces 2 features:

1. DISCOURSE_COMPRESS_ANON_CACHE (true|false, default false): this allows
you to optionally compress the anon cache body entries in Redis, can be
useful for high load sites with Redis that lives on a separate server to
to webs

2. DISCOURSE_ANON_CACHE_STORE_THRESHOLD (default 2), only pop entries into
redis if we observe them more than N times. This avoids situations where
a crawler can walk a big pile of topics and store them all in Redis never
to be used. Our default anon cache time for topics is only 60 seconds. Anon
cache is in place to avoid the "slashdot" effect where a single topic is
hit by 100s of people in one minute.

spec/components/middleware/anonymous_cache_spec.rb

@@ -129,6 +129,24 @@ describe Middleware::AnonymousCache::Helper do
       crawler.clear_cache
     end
 
+    before do
+      global_setting :anon_cache_store_threshold, 1
+    end
+
+    it "compresses body on demand" do
+      global_setting :compress_anon_cache, true
+
+      payload = "x" * 1000
+      helper.cache([200, { "HELLO" => "WORLD" }, [payload]])
+
+      helper = new_helper("ANON_CACHE_DURATION" => 10)
+      expect(helper.cached).to eq([200, { "X-Discourse-Cached" => "true", "HELLO" => "WORLD" }, [payload]])
+
+      # depends on i7z implementation, but lets assume it is stable unless we discover
+      # otherwise
+      expect($redis.get(helper.cache_key_body).length).to eq(16)
+    end
+
     it "handles brotli switching" do
       helper.cache([200, { "HELLO" => "WORLD" }, ["hello ", "my world"]])