(VDB-570) Handle duplicate storage diffs

- If processing a new diff for a row that already exists in the DB,
  ignore the error without logging or queueing
- If processing a queued diff for a row that already exists, remove
  it from the queue

libraries/shared/watcher/storage_watcher_test.go

@@ -109,6 +109,18 @@ var _ = Describe("Storage Watcher", func() {
 				close(done)
 			})
 
+			It("does not queue row if transformer execution fails because row already exists", func(done Done) {
+				mockTransformer.ExecuteErr = utils.ErrRowExists
+
+				go storageWatcher.Execute(rows, errs, time.Hour)
+
+				Expect(<-errs).To(BeNil())
+				Consistently(func() bool {
+					return mockQueue.AddCalled
+				}).Should(BeFalse())
+				close(done)
+			})
+
 			It("queues row for later processing if transformer execution fails", func(done Done) {
 				mockTransformer.ExecuteErr = fakes.FakeError
 
@@ -187,6 +199,17 @@ var _ = Describe("Storage Watcher", func() {
 				close(done)
 			})
 
+			It("deletes row from queue if transformer execution errors because row already exists", func(done Done) {
+				mockTransformer.ExecuteErr = utils.ErrRowExists
+
+				go storageWatcher.Execute(rows, errs, time.Nanosecond)
+
+				Eventually(func() int {
+					return mockQueue.DeletePassedId
+				}).Should(Equal(row.Id))
+				close(done)
+			})
+
 			It("logs error if deleting persisted row fails", func(done Done) {
 				mockQueue.DeleteErr = fakes.FakeError
 				tempFile, fileErr := ioutil.TempFile("", "log")