fix trailing checksum

[sbiscigl]

Issue #, if available:
issues/3132

Description of changes:
There was a nasty bit of undefined behavior in the aws-chunked logic for curl. The original implementation assumed that curl would always provide us with a chunk larger than 8kb to per ReadFunc callback. That is simply not true, and it will cause requests where curl calls readfunc with less than 8kb to fail consistently.This buffers the input stream and outputs to curl valid sized chunks, specifically 64kb chunks as S3 specifies as their preference

The chunk size must be at least 8 KB. We recommend a chunk size of a least 64 KB for better performance. This chunk size applies to all chunks except the last one. The last chunk you send can be smaller than 8 KB. If your payload is small and can fit into one chunk, then it can be smaller than the 8 KB.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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