Forcefully shuts down this TransferManager instance - currently executing transfers will not be allowed to finish. Callers should use this method when they either:
- have already verified that their transfers have completed by checking each transfer's state
- need to exit quickly and don't mind stopping transfers before they complete.
Callers should also remember that uploaded parts from an interrupted upload may not always be automatically cleaned up, but callers can use {@link #abortMultipartUploads(String,Date)} to clean up any uploadparts.