Write consistency check

[martindurant]

Counterpart to fsspec/gcsfs#20

Note that for S3, the ETag is an MD5 hash for simple uploads, and an MD5 hash of concatenated MD5 hashes in the case of multi-part uploads https://stackoverflow.com/questions/6591047/etag-definition-changed-in-amazon-s3

[TomAugspurger]

Debugging a separate issue, I came across this same thing: https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/s3.html#S3.Client.upload_part

The base64-encoded 128-bit MD5 digest of the part data. This parameter is auto-populated when using the command from the CLI. This parameted is required if object lock parameters are specified

cc @jmunroe, this would likely have helped with the partial uploads you were seeing when uploading that Zarr dataset.

API-wise, I need to get a flag like validate_write=False/True to _upload_chunk. This would probably be a parameter when creating the S3File, but I'm not sure about that.