VDB-104 improve lightsync ui (#125)

* Return PopulateMissingHeaders early if the sync is at the head of the chain

* Squelch logging if no blocks to sync

* Fix broken test

* Refactor repository tests

pkg/fakes/mock_header_repository.go

@@ -12,6 +12,7 @@ type MockHeaderRepository struct {
 	createOrUpdateHeaderPassedBlockNumbers []int64
 	createOrUpdateHeaderReturnID           int64
 	missingBlockNumbers                    []int64
+	headerExists                           bool
 }
 
 func NewMockHeaderRepository() *MockHeaderRepository {
@@ -44,6 +45,14 @@ func (repository *MockHeaderRepository) MissingBlockNumbers(startingBlockNumber,
 	return repository.missingBlockNumbers
 }
 
+func (repository *MockHeaderRepository) HeaderExists(blockNumber int64) (bool, error) {
+	return repository.headerExists, nil
+}
+
+func (repository *MockHeaderRepository) SetHeaderExists(headerExists bool) {
+	repository.headerExists = headerExists
+}
+
 func (repository *MockHeaderRepository) AssertCreateOrUpdateHeaderCallCountAndPassedBlockNumbers(times int, blockNumbers []int64) {
 	Expect(repository.createOrUpdateHeaderCallCount).To(Equal(times))
 	Expect(repository.createOrUpdateHeaderPassedBlockNumbers).To(Equal(blockNumbers))