bulkdata: write _format file atomically, to reduce chance of corruption

Some databases were seeing _format truncated to 0 bytes, after crashing
soon after a new stream was created (e.g. during decimation).

nilmdb/server/bulkdata.py

@@ -293,8 +293,8 @@ class Table():
             "layout": layout,
             "version": 3
         }
-        with open(os.path.join(root, b"_format"), "wb") as f:
-            pickle.dump(fmt, f, 2)
+        nilmdb.utils.atomic.replace_file(
+            os.path.join(root, b"_format"), pickle.dumps(fmt, 2))
 
     # Normal methods
     def __init__(self, root, initial_nrows=0):