Update util.writeFile to write to tmp file before rename

[knolleary]

• Bugfix (non-breaking change which fixes an issue)

Proposed changes

The Local Filesystem storage module has a utility function used to write files - this is used in a number of places such as the flow/credential files and the runtime settings (.config.json).

The existing code does the follow:

1. renames the existing file to a backup file (if the caller provided a backup file name)
2. writes the new content to the desired file
3. tries to sync the file descriptor to get the writes flushed

This means there is a window of time where the original file has been renamed and the new content has not yet been written to the file.

I have seen a small number of times when using grunt dev (which will cause the node process to be killed whenever I modify a file) that the .config.json file is corrupted and the runtime settings are lost.

I cannot reliably reproduce it, but I'm pretty sure its related to killing Node-RED whilst its starting up.

This PR modifies the logic around the writeFile function. It now does:

1. copies the existing file to a backup file (if the caller provided a backup file name)
2. writes the new content to a temporary file (the filename is <file>.$$$)
3. flushes the file descriptor
4. renames the temporary file to the desired file.

That means the file has valid content through-out the process - as the final rename will be an atomic fs action.