Issue with config file being overwritten

[jamespcole]

Hi, first off, thanks for this project, it's very cool. I have noticed an issue with reads/writes to the config file in multi-threaded situations though.

If you take a look at line 299 here https://github.com/adewes/blitzdb/blob/master/blitzdb/backends/file/backend.py#L299

save_config appears to always get called regardless of the overwrite_config value, as far as I can tell it gets called everytime the config is loaded. Because the issue is sporadic it's difficult to reproduce with an example but the symptoms are an error message like this:

  File "/home/vagrant/.pyvenv/rcvenv/src/blitzdb-master/blitzdb/backends/file/backend.py", line 111, in __init__
    self.load_config(config, overwrite_config)
  File "/home/vagrant/.pyvenv/rcvenv/src/blitzdb-master/blitzdb/backends/file/backend.py", line 285, in load_config
    self._config = JsonSerializer.deserialize(config_file.read())
  File "/home/vagrant/.pyvenv/rcvenv/src/blitzdb-master/blitzdb/backends/file/serializers.py", line 33, in deserialize
    return json.loads(data.decode('utf-8'))
  File "/usr/lib/python3.4/json/__init__.py", line 318, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.4/json/decoder.py", line 343, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.4/json/decoder.py", line 361, in raw_decode
    raise ValueError(errmsg("Expecting value", s, err.value)) from None
ValueError: Expecting value: line 1 column 1 (char 0)

I'm pretty certain that it is due to the file being written and read at the same time. It should be a simple fix, so if I can, I'll issue a PR in the next few days if I can find the time.