core: add Close() to blockchainer, implement it to properly close chain

Before it the deferred function in Run() was actually never able to properly
close the Store, so we weren't synching the latest state to the disk.

pkg/core/blockchain_test.go

@@ -155,3 +155,23 @@ func TestGetTransaction(t *testing.T) {
 		assert.NoError(t, bc.persist())
 	}
 }
+
+func TestClose(t *testing.T) {
+	defer func() {
+		r := recover()
+		assert.NotNil(t, r)
+	}()
+	bc := newTestChain(t)
+	blocks := makeBlocks(10)
+	for i := 0; i < len(blocks); i++ {
+		require.NoError(t, bc.AddBlock(blocks[i]))
+	}
+	bc.Close()
+	// It's a hack, but we use internal knowledge of MemoryStore
+	// implementation which makes it completely unusable (up to panicing)
+	// after Close().
+	_ = bc.store.Put([]byte{0}, []byte{1})
+
+	// This should never be executed.
+	assert.Nil(t, t)
+}