Change JsonFileConfigStore

[aloslider]

Trying to resolve issue #128, so here's my thoughts and changes.

• JsonFileConfigStore didn't check whether the filepath exists (it checked only in WriteJsonFile() method), so it could lead to store initialization with not really existing configuration source (I assumed that it can be created when there is an existing configuration file), but now it will, and if it doesn't - exception will be thrown.
• File.Replace() method can throw an exception, because the project folder could exist on different volume from the system one where the temporary files get created within the Temp folder, so it cannot replace files. Instead, it is possible to use File.Copy() + File.Delete() methods as they do not throw such volumes depended exceptions.
• There was some sophisticated logic in copying and overwriting data files (which I actually could misunderstand), so it was simplified to this scenario:
  1. Create temporary data file with new values to overwrite in Temp folder
  2. Create backup file of old data file (if it's not possible, exception will be thrown, since backup file is important I guess)
  3. Try to overwrite old data file with the new one
  4. If something gone wrong, restore data with backup
  5. Remove temporary and backup files
• The Path.GetTempFileName() method is not safe, since it uses some old API's and generates only four hex characters ID. Instead, now temporary *.tmp file full path is generated with Path.GetTempPath() and Guid.NewGuid() parts.

I tried to fix only my issue, but there could be probably more changes to this class, e.g. now ReadJsonFile() method doesn't have to check if file exists since it was already done in the constructor. Anyway, hope some of these changes are useful.

[eveloki]

It's been three months, why hasn't anyone worked on this change? Has this project been abandoned? I have this problem and he needs to be fixed。

[WithAnOrchid]

Seems this project needs urgent care, except for this bug, reading a large amount of config from JSON is quite slow.