Add buckets caching to GcsFS

[meandnano]

Without it each files interaction on GCS results in an unnecessary GetBucket request.

[CLAassistant]

All committers have signed the CLA.

[mvrhov]

@sagikazarmark can you please find time and review/merge gcsfs merge requests #424, #457, #568. They are pretty small but still significant. We are trying to get rid of various repo forks as they are getting annoying.

[spf13]

Closing — superseded by #568, which is a strict superset: same bucket caching, plus a proper sync.RWMutex protecting the cache map (avoiding the data race your PR would have introduced with concurrent access), plus configurable upload chunk size. Thanks for identifying the missing initialization — that was a real bug.