S3transfer

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* bugfix:Cntrl-C: Fix issue of hangs when Cntrl-C happens for many queued transfers
* feature:cancel: Expose messages for cancels

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* feature:chunksize: Automatically adjust the chunksize if it doesn't meet S3s requirements.
* bugfix:Download: Add support for downloading to special UNIX file by name

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* feature:delete: Add a ``.delete()`` method to the transfer manager.
* bugfix:seekable upload: Fix issue where seeked position of seekable file for a nonmultipart upload was not being taken into account.

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* bugfix:download: Patch memory leak related to unnecessarily holding onto futures for downloads.

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* bugfix:deadlock: Fix deadlock issue described here: https://bugs.python.org/issue20319 with using concurrent.futures.wait

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* feature:copy: Add support for managed copies.
* feature:download: Add support for downloading to a filename, seekable file-like object, and nonseekable file-like object.
* feature:general: Add ``TransferManager`` class. All public functionality for ``s3transfer`` is exposed through this class.
* feature:subscribers: Add subscriber interface. Currently supports on_queued, on_progress, and on_done status changes.
* feature:upload: Add support for uploading a filename, seekable file-like object, and nonseekable file-like object.