DEV: use Discourse.cache over Rails.cache

Discourse.cache is a more consistent method to use and offers clean fallback
if you are skipping redis

This is part of a larger change that both optimizes Discoruse.cache and omits
use of setex on $redis in favor of consistently using discourse cache

Bench does reveal that use of Rails.cache and Discourse.cache is 1.25x slower
than redis.setex / get so a re-implementation will follow prior to porting

script/benchmarks/cache/bench.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+require 'benchmark/ips'
+require File.expand_path('../../../../config/environment', __FILE__)
+
+Benchmark.ips do |x|
+
+  x.report("redis setex string") do |times|
+    while times > 0
+      $redis.setex("test_key", 60, "test")
+      times -= 1
+    end
+  end
+
+  x.report("redis setex marshal string") do |times|
+    while times > 0
+      $redis.setex("test_keym", 60, Marshal.dump("test"))
+      times -= 1
+    end
+  end
+
+  x.report("Discourse cache string") do |times|
+    while times > 0
+      Discourse.cache.write("test_key", "test", expires_in: 60)
+      times -= 1
+    end
+  end
+
+  x.report("Rails cache string") do |times|
+    while times > 0
+      Rails.cache.write("test_key_rails", "test", expires_in: 60)
+      times -= 1
+    end
+  end
+
+  x.compare!
+end
+
+Benchmark.ips do |x|
+  x.report("redis get string") do |times|
+    while times > 0
+      $redis.get("test_key")
+      times -= 1
+    end
+  end
+
+  x.report("redis get string marshal") do |times|
+    while times > 0
+      Marshal.load($redis.get("test_keym"))
+      times -= 1
+    end
+  end
+
+  x.report("Discourse read cache string") do |times|
+    while times > 0
+      Discourse.cache.read("test_key")
+      times -= 1
+    end
+  end
+
+  x.report("Rails read cache string") do |times|
+    while times > 0
+      Rails.cache.read("test_key_rails")
+      times -= 1
+    end
+  end
+
+  x.compare!
+end
+
+# Comparison:
+#   redis setex string:    13250.0 i/s
+# redis setex marshal string:    12866.4 i/s - same-ish: difference falls within error
+# Discourse cache string:    10443.0 i/s - 1.27x  slower
+#   Rails cache string:    10367.9 i/s - 1.28x  slower
+
+# Comparison:
+#     redis get string:    13147.4 i/s
+# redis get string marshal:    12789.2 i/s - same-ish: difference falls within error
+# Rails read cache string:    10486.4 i/s - 1.25x  slower
+# Discourse read cache string:    10457.1 i/s - 1.26x  slower