Improve the block transfer logic

- Fix a bug where the blockchain could not be synced completely when
  block transfers happened in parallel
- Sum up multiple received blocks in the log output

node.py

@@ -198,6 +198,7 @@ def transfer_block(addr, node, receive_observer, b):
                 if existing_block is not None:
                     if existing_block.valid:
                         break
+                    block_list.append(request_block_hash)
                     request_block_hash = existing_block.previous_hash
                     if request_block_hash == 32*b"\0":
                         break
@@ -212,11 +213,16 @@ def transfer_block(addr, node, receive_observer, b):
             request_block_hash = b.get_block(block_hash).previous_hash
             if request_block_hash == 32*b"\0":
                 break
+        new_block_count = 0
         for block_hash in reversed(block_list):
             if not b.get_block(block_hash).validate(b):
-                return
+                break
             if b.set_latest_block(block_hash):
-                log("Got a new block")
+                new_block_count += 1
+        if new_block_count == 1:
+            log("Got a new block")
+        elif new_block_count > 1:
+            log(f"Got {new_block_count} new blocks")
     except NoReponseException:
         pass