DEV: Implement a faster Discourse.cache

This is a bottom up rewrite of Discourse cache to support faster performance
and a limited surface area.

ActiveSupport::Cache::Store accepts many options we do not use, this partial
implementation only picks the bits out that we do use and want to support.

Additionally params are named which avoids typos such as "expires_at" vs "expires_in"

This also moves a few spots in Discourse to use Discourse.cache over setex
Performance of setex and Discourse.cache.write is similar.

spec/components/cache_spec.rb

@@ -9,6 +9,12 @@ describe Cache do
     Cache.new
   end
 
+  it "supports exist?" do
+    cache.write("testing", 1.1)
+    expect(cache.exist?("testing")).to eq(true)
+    expect(cache.exist?(SecureRandom.hex)).to eq(false)
+  end
+
   it "supports float" do
     cache.write("float", 1.1)
     expect(cache.read("float")).to eq(1.1)
@@ -36,10 +42,14 @@ describe Cache do
   end
 
   it "can delete correctly" do
+    cache.delete("key")
+
     cache.fetch("key", expires_in: 1.minute) do
       "test"
     end
 
+    expect(cache.fetch("key")).to eq("test")
+
     cache.delete("key")
     expect(cache.fetch("key")).to eq(nil)
   end
@@ -69,6 +79,7 @@ describe Cache do
     r = cache.fetch "key" do
       "bob"
     end
+
     expect(r).to eq("bob")
   end