Add support for aws_chunked with s3v4.

[timuralp]

When uploading to S3 from a stream, it would be useful to opt into S3
aws_chunked uploads with v4 signatures. The mechanism is documented
here:
http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming.html

The feature is implemented through a new auth class --
S3SigV4ChunkedAuth. The class wraps the body of the request with a
ChunkedUploadWrapper instance. It takes care of producing fixed size
upload chunks and computing the required signatures for each one.

To opt into this scheme, the caller is required to set the aws_chunked
option for client configuration and it will only be used with the
PutObject operations.

Fixes #995

[timuralp]

I wasn't sure if this is the right approach to this problem and the PR doesn't have any unit tests at the moment. If there is agreement that this make sense or guidance on how to rework it, I'm happy to work further on the PR and add the tests.

[codecov-io]

Codecov Report

Merging #996 into develop will decrease coverage by 1.35%.
The diff coverage is 89.14%.

@@             Coverage Diff             @@
##           develop     #996      +/-   ##
===========================================
- Coverage    98.03%   96.68%   -1.36%     
===========================================
  Files           45       44       -1     
  Lines         7345     7264      -81     
===========================================
- Hits          7201     7023     -178     
- Misses         144      241      +97

Impacted Files	Coverage Δ
botocore/signers.py	97.25% <80%> (-1.19%)	⬇️
botocore/auth.py	96.29% <89.51%> (-2.01%)	⬇️
botocore/configloader.py	75.94% <0%> (-24.06%)	⬇️
botocore/compat.py	68.14% <0%> (-23.71%)	⬇️
botocore/translate.py	92.68% <0%> (-7.32%)	⬇️
botocore/handlers.py	94.13% <0%> (-2.19%)	⬇️
botocore/docs/sharedexample.py	97.16% <0%> (-1.42%)	⬇️
botocore/docs/bcdoc/style.py	94.05% <0%> (-1.17%)	⬇️
botocore/paginate.py	96.59% <0%> (-0.61%)	⬇️
... and 26 more

---

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[jamesls]

Linking issues, boto3 tracking issue: boto/boto3#751

Pending discussion over on the issue.

[github-actions]

Greetings! It looks like this issue hasn’t been active in longer than one year. We encourage you to check if this is still an issue in the latest release. Because it has been longer than one year since the last update on this, and in the absence of more information, we will be closing this issue soon. If you find that this is still a problem, please feel free to provide a comment to prevent automatic closure, or if the issue is already closed, please feel free to reopen it.

[timuralp]

I don't know if there is anything I could've done to draw more attention to this or generate discussion. In light of lack of desire to discuss this feature, it probably makes sense to close it.