Fix printing bugs when using no-overwrite flag

[aemous]

Description of changes:

• Fixed bug when using --no-overwrite parameter where the last character printed to CLI would be a carriage return (\r), which would cause incompatibility with some terminal program (e.g. Windows Powershell).
  • Keep track of files skipped for determining whether there is progress remaining in ResultPrinter.
  • Update files remaining progress to take into account files skipped.
  • Add support for results that do not print newlines (e.g. new SkipFileResult).
  • Modify result handling in ResultPrinter so that they can opt-out of resetting progress_length to 0. Useful for result handlers that do not print newlines (e.g. new SkipFileResult).

Description of tests:

• Manually tested changes on Windows Powershell, with uploads, bucket-to-bucket copies, and downloads. Verified the bug is fixed in all three cases.

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[kdaily]

This LGTM. I ran it through the test suite after pulling in the latest changes from v2 and that was OK. I think we should add a test to exercise the newly added SkipFileResult?

[kdaily]

LGTM!

Lets make sure to clean up the commit history or squash before merging, preferably getting some of the details from the description on this PR into the final commit(s). Thanks!