[aws-cpp-sdk-core]: do not flush ResponseStream before deletion

[grrtrr]

Flushing the ResponseStream before deletion causes undefined behaviour and segmentation faults, since the ResponseStream is destructed after the actual download has completed.

Resolves #58, #2319 and in part #1732.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
  Tested with a reproducing command (which reliably produced the segmentation fault described in [aws-cpp-sdk-core] undefined behaviour when flushing ResponseStream before deletion #2319) that the condition no longer occurs after applying this change.
• 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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[SergeyRyabinin]

Hi @grrtrr,

I have merged your commit as a part of #2351 (sorry I forgot to merge your PR first). I have made some additional changes to this class (to register callback on the underlying stream) and the completion notification logic (flush stream in TransferManager before updating the status).
Thank you again for a great finding and analysis.

Best regards,
Sergey