Added concern for a faster "cache_key" methodfaster-cache-keys

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.

1 files changed, 17 insertions, 0 deletions

diff --git a/spec/models/concerns/faster_cache_keys_spec.rb b/spec/models/concerns/faster_cache_keys_spec.rb
new file mode 100644
index 00000000000..8d3f94267fa
--- /dev/null
+++ b/spec/models/concerns/faster_cache_keys_spec.rb
@@ -0,0 +1,17 @@
+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