Do not rewrite the blockchain file at startup

blockchain.py

@@ -200,8 +200,8 @@ class Blockchain:
         except FileNotFoundError:
             pass
         if last_valid is not None:
-            self.set_latest_block(last_valid.own_hash)
-    def set_latest_block(self, block_hash):
+            self.set_latest_block(last_valid.own_hash, persist = False)
+    def set_latest_block(self, block_hash, persist = True):
         new_block = self.get_block(block_hash)
         assert new_block is not None
         assert new_block.valid
@@ -220,6 +220,7 @@ class Blockchain:
                 if self.__latest_block_hash != latest_block_hash:
                     continue
                 self.__latest_block_hash = block_hash
+                if persist:
                     self.__persist_block_update(latest_block, new_block)
             return True
     def __persist_block_update(self, old_block, new_block):