Finish GSbucket session handler implementation

This PR adds missing write(), read(), destroy() functions for GSbucket session handler.

I've chosen .tmp as the extension for the session file.

Now this is working perfectly fine!

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On a side note I think this as well as the cache handler would benefit from having an option to define the folders in which all data is stored (session and cache currently) within a bucket. A benefit is that when the folder name is empty it would mean that admin wants to store all data in top-level folder and we won't need the additional check if session/cache folders exists => 1 less gs request to process