Add ability to define minimal time between job executions to support multiple corno nodes, so two different nodes will not execute the same job

spec/job_spec.rb

@@ -36,6 +36,32 @@ describe Crono::Job do
       failing_job.perform.join
       expect(failing_job.healthy).to be false
     end
+
+    it 'should execute one' do
+      job.execution_interval = 5.minutes
+
+      expect(job).to receive(:perform_job).once
+      job.perform.join
+      thread = job.perform.join
+      expect(thread).to be_stop
+    end
+
+    it 'should execute twice' do
+      job.execution_interval = 0.minutes
+
+      test_preform_job_twice
+    end
+
+    it 'should execute twice without initialize execution_interval' do
+      test_preform_job_twice
+    end
+
+    def test_preform_job_twice
+      expect(job).to receive(:perform_job).twice
+      job.perform.join
+      thread = job.perform.join
+      expect(thread).to be_stop
+    end
   end
 
   describe '#description' do

spec/performer_proxy_spec.rb

@@ -5,4 +5,17 @@ describe Crono::PerformerProxy do
     expect(Crono.scheduler).to receive(:add_job).with(kind_of(Crono::Job))
     Crono.perform(TestJob).every(2.days, at: '15:30')
   end
+
+  it 'should set execution interval' do
+    allow(Crono).to receive(:scheduler).and_return(Crono::Scheduler.new)
+    expect_any_instance_of(Crono::Job).to receive(:execution_interval=).with(0.minutes).once
+    expect_any_instance_of(Crono::Job).to receive(:execution_interval=).with(10.minutes).once
+    Crono.perform(TestJob).every(2.days, at: '15:30').once_per 10.minutes
+  end
+
+  it 'do nothing when job not initalized' do
+    expect_any_instance_of(Crono::Job).not_to receive(:execution_interval=)
+    expect_any_instance_of(described_class).to receive(:once_per)
+    Crono.perform(TestJob).once_per 10.minutes
+  end
 end