Add lint checks: gofmt, goimports, golint

- gofmt - Gofmt checks whether code was gofmt-ed. By default this tool runs
  with -s option to check for code simplification
- goimports - Goimports does everything that gofmt does. Additionally it checks
  unused imports
- golint - Golint differs from gofmt. Gofmt reformats Go source code, whereas
  golint prints out style mistakes
    - checks the uncommented exported functions & types

Update the code to fix the lint checks.

coordinator/coordinator_test.go

@@ -46,7 +46,7 @@ func newTestModules(t *testing.T) (*txselector.TxSelector, *batchbuilder.BatchBu
 func TestCoordinator(t *testing.T) {
 	txsel, bb := newTestModules(t)
 
-	conf := CoordinatorConfig{
+	conf := Config{
 		LoopInterval: 100 * time.Millisecond,
 	}
 	hdb := &historydb.HistoryDB{}