Allow size= in open()

[martindurant]

Fixes #794

@jrbourbeau , do you want to test? I'll think about how to test this in CI.

[jrbourbeau]

Thanks @martindurant! I'll test this out

Naively, looking at the PR diff, we may still be triggering an s3.info call here https://github.com/fsspec/s3fs/pull/797/files#diff-044e3dd45e47667275a210a6ac0033c31687eaea7d4e33e3b21b34baa1a050daR2083 (I could be wrong though)

[martindurant]

That's only for buckets with versioning. I don't suppose I need to change too many code paths in one go here, but I has better yet add the docstring for size=

[martindurant]

# after any initial call which does SSL and checks credentials. 
In [4]: %time f = fs.open("mymdtemp/icesat2-4.01.json")
CPU times: user 4.07 ms, sys: 1.76 ms, total: 5.83 ms
Wall time: 57.3 ms

In [5]: fs.invalidate_cache()

In [6]: %time f = fs.open("mymdtemp/icesat2-4.01.json", size=100)
CPU times: user 110 µs, sys: 0 ns, total: 110 µs
Wall time: 113 µs

[jrbourbeau]

Thanks for this update @martindurant, I'm also seeing performance improvements when opening with size= specified