blockchain: add waiting persist goroutine before the Shutdown

The `persist` goroutine could write logs after the blockchain service
shutdown that leads to tests failure.

Close #3217

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/core/blockchain.go

@@ -159,6 +159,7 @@ type Blockchain struct {
 	// Stop synchronization mechanisms.
 	stopCh      chan struct{}
 	runToExitCh chan struct{}
+	wgPersist   sync.WaitGroup
 	// isRunning denotes whether blockchain routines are currently running.
 	isRunning atomic.Value
 
@@ -1441,6 +1442,7 @@ func (bc *Blockchain) notificationDispatcher() {
 // Close stops Blockchain's internal loop, syncs changes to persistent storage
 // and closes it. The Blockchain is no longer functional after the call to Close.
 func (bc *Blockchain) Close() {
+	bc.wgPersist.Wait()
 	// If there is a block addition in progress, wait for it to finish and
 	// don't allow new ones.
 	bc.addLock.Lock()
@@ -2113,6 +2115,8 @@ func (bc *Blockchain) persist(isSync bool) (time.Duration, error) {
 		persisted int
 		err       error
 	)
+	bc.wgPersist.Add(1)
+	defer bc.wgPersist.Done()
 
 	if isSync {
 		persisted, err = bc.dao.PersistSync()