Add :none strategy for deduplicating Sidekiq jobs

Allows a job not to be deduplicated even though it is idempotent:

```ruby
  class DummyWorker
    include ApplicationWorker

    deduplicate :none
    idempotent!

    # ...
  end
```

lib/gitlab/sidekiq_middleware/duplicate_jobs/strategies.rb

@@ -7,7 +7,8 @@ module Gitlab
         UnknownStrategyError = Class.new(StandardError)
 
         STRATEGIES = {
-          until_executing: UntilExecuting
+          until_executing: UntilExecuting,
+          none: None
         }.freeze
 
         def self.for(name)

lib/gitlab/sidekiq_middleware/duplicate_jobs/strategies/none.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module SidekiqMiddleware
+    module DuplicateJobs
+      module Strategies
+        # This strategy will never deduplicate a job
+        class None
+          def initialize(_duplicate_job)
+          end
+
+          def schedule(_job)
+            yield
+          end
+
+          def perform(_job)
+            yield
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/sidekiq_middleware/duplicate_jobs/strategies/none_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+RSpec.describe Gitlab::SidekiqMiddleware::DuplicateJobs::Strategies::None do
+  let(:fake_duplicate_job) do
+    instance_double(Gitlab::SidekiqMiddleware::DuplicateJobs::DuplicateJob)
+  end
+
+  subject(:strategy) { described_class.new(fake_duplicate_job) }
+
+  describe '#schedule' do
+    it 'yields without checking for duplicates', :aggregate_failures do
+      expect(fake_duplicate_job).not_to receive(:scheduled?)
+      expect(fake_duplicate_job).not_to receive(:duplicate?)
+      expect(fake_duplicate_job).not_to receive(:check!)
+
+      expect { |b| strategy.schedule({}, &b) }.to yield_control
+    end
+  end
+
+  describe '#perform' do
+    it 'does not delete any locks before executing', :aggregate_failures do
+      expect(fake_duplicate_job).not_to receive(:delete!)
+
+      expect { |b| strategy.perform({}, &b) }.to yield_control
+    end
+  end
+end

spec/lib/gitlab/sidekiq_middleware/duplicate_jobs/strategies_spec.rb

@@ -8,6 +8,10 @@ RSpec.describe Gitlab::SidekiqMiddleware::DuplicateJobs::Strategies do
       expect(described_class.for(:until_executing)).to eq(described_class::UntilExecuting)
     end
 
+    it 'returns the right class for `none`' do
+      expect(described_class.for(:none)).to eq(described_class::None)
+    end
+
     it 'raises an UnknownStrategyError when passing an unknown key' do
       expect { described_class.for(:unknown) }.to raise_error(described_class::UnknownStrategyError)
     end