Make Metadata store account for IOExceptions

[useless5771]

Updated GistMetadataStore and GistCommentStore so they handle possible IOExceptions thrown (and unexpected failures) during updates to JSON files.

The implementation (of both) is as follows:

• they always write to a working copy of a file
• if writing to a working copy fails, it is deleted.
• if there happens to be an exception thrown during replacement of the main copy with the working copy, the state is recovered from the working copy
• if it happens that both - working copy and main file is there, working copy is ignored.

I covered the changes with unit tests, however I haven't performed any integration tests with RCloud, as I don't have RCloud configured to work with rcloud-gist-service (yet).

FIX #2

[gordonwoodhull]

@s-u, we should review and merge this.