rewrite some checksum related code

[TingDaoK]

Issue #, if available:

• Get into this rabbit hole from removing body_user_callback_after_checksum to support write into file directly

Description of changes:

• No functionality change, beside I modified the request response header directly instead of making a copy.
• Remove the *_after_checksum callbacks, just invoke the right function from the right place
• we don't need to make a copy of headers before we send it to the user, we are not using the response headers anymore afterwards
• reuse some code instead of copy/paste

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.17204% with 11 lines in your changes missing coverage. Please review.

Project coverage is 89.44%. Comparing base (94e3342) to head (6698fc9).

Files	Patch %	Lines
source/s3_meta_request.c	89.79%	5 Missing :warning:
source/s3_util.c	88.46%	3 Missing :warning:
source/s3_auto_ranged_get.c	66.66%	2 Missing :warning:
source/s3_default_meta_request.c	88.88%	1 Missing :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) ```diff @@ Coverage Diff @@ ## main #443 +/- ## ========================================== - Coverage 89.62% 89.44% -0.19% ========================================== Files 20 20 Lines 6092 6080 -12 ========================================== - Hits 5460 5438 -22 - Misses 632 642 +10 ``` | [Files](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) | Coverage Δ | | |---|---|---| | [source/s3\_auto\_ranged\_put.c](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?src=pr&el=tree&filepath=source%2Fs3_auto_ranged_put.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3MzX2F1dG9fcmFuZ2VkX3B1dC5j) | `92.78% <100.00%> (-0.02%)` | :arrow_down: | | [source/s3\_default\_meta\_request.c](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?src=pr&el=tree&filepath=source%2Fs3_default_meta_request.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3MzX2RlZmF1bHRfbWV0YV9yZXF1ZXN0LmM=) | `95.23% <88.88%> (-0.52%)` | :arrow_down: | | [source/s3\_auto\_ranged\_get.c](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?src=pr&el=tree&filepath=source%2Fs3_auto_ranged_get.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3MzX2F1dG9fcmFuZ2VkX2dldC5j) | `97.23% <66.66%> (-0.55%)` | :arrow_down: | | [source/s3\_util.c](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?src=pr&el=tree&filepath=source%2Fs3_util.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3MzX3V0aWwuYw==) | `97.16% <88.46%> (-0.78%)` | :arrow_down: | | [source/s3\_meta\_request.c](https://app.codecov.io/gh/awslabs/aws-c-s3/pull/443?src=pr&el=tree&filepath=source%2Fs3_meta_request.c&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3MzX21ldGFfcmVxdWVzdC5j) | `92.77% <89.79%> (-0.69%)` | :arrow_down: |