Added concern for a faster "cache_key" method

This concern provides an optimized/simplified version of the "cache_key"
method. This method is about 9 times faster than the default "cache_key"
method.

The produced cache keys _are_ different from the previous ones but this
is worth the performance improvement. To showcase this I set up a
benchmark (using benchmark-ips) that compares FasterCacheKeys#cache_key
with the regular cache_key. The output of this benchmark was:

    Calculating -------------------------------------
               cache_key     4.825k i/100ms
          cache_key_fast    21.723k i/100ms
    -------------------------------------------------
               cache_key     59.422k (± 7.2%) i/s -    299.150k
          cache_key_fast    543.243k (± 9.2%) i/s -      2.694M

    Comparison:
          cache_key_fast:   543243.4 i/s
               cache_key:    59422.0 i/s - 9.14x slower

To see the impact on real code I applied these changes and benchmarked
Issue#referenced_merge_requests. For an issue referencing 10 merge
requests these changes shaved off between 40 and 60 milliseconds.

CHANGELOG

@@ -25,6 +25,7 @@ v 8.11.0 (unreleased)
   - Limit git rev-list output count to one in forced push check
   - Clean up unused routes (Josef Strzibny)
   - Add green outline to New Branch button. !5447 (winniehell)
+  - Optimize generating of cache keys for issues and notes
   - Improve performance of syntax highlighting Markdown code blocks
   - Update to gitlab_git 10.4.1 and take advantage of preserved Ref objects
   - Remove delay when hitting "Reply..." button on page with a lot of discussions

app/models/concerns/faster_cache_keys.rb

+module FasterCacheKeys
+  # A faster version of Rails' "cache_key" method.
+  #
+  # Rails' default "cache_key" method uses all kind of complex logic to figure
+  # out the cache key. In many cases this complexity and overhead may not be
+  # needed.
+  #
+  # This method does not do any timestamp parsing as this process is quite
+  # expensive and not needed when generating cache keys. This method also relies
+  # on the table name instead of the cache namespace name as the latter uses
+  # complex logic to generate the exact same value (as when using the table
+  # name) in 99% of the cases.
+  def cache_key
+    "#{self.class.table_name}/#{id}-#{read_attribute_before_type_cast(:updated_at)}"
+  end
+end

app/models/issue.rb

@@ -7,6 +7,7 @@ class Issue < ActiveRecord::Base
   include Sortable
   include Taskable
   include Spammable
+  include FasterCacheKeys
 
   DueDateStruct = Struct.new(:title, :name).freeze
   NoDueDate     = DueDateStruct.new('No Due Date', '0').freeze

app/models/note.rb

@@ -5,6 +5,7 @@ class Note < ActiveRecord::Base
   include Mentionable
   include Awardable
   include Importable
+  include FasterCacheKeys
 
   # Attribute containing rendered and redacted Markdown as generated by
   # Banzai::ObjectRenderer.

spec/models/concerns/faster_cache_keys_spec.rb

+require 'spec_helper'
+
+describe FasterCacheKeys do
+  describe '#cache_key' do
+    it 'returns a String' do
+      # We're using a fixed string here so it's easier to set an expectation for
+      # the resulting cache key.
+      time = '2016-08-08 16:39:00+02'
+      issue = build(:issue, updated_at: time)
+      issue.extend(described_class)
+
+      expect(issue).to receive(:id).and_return(1)
+
+      expect(issue.cache_key).to eq("issues/1-#{time}")
+    end
+  end
+end