Adds abort multipart upload

According to http://docs.aws.amazon.com/AmazonS3/latest/API/mpUploadAbort.html

This also adds synchronization for concurrent completion or abortion
of a multipart upload, where the request that came "last" will receive an error
response. This synchronization at least guarantees a clean state, i.e. it prevents
that concurrent abortion or completion result in an undefined state. This issue
actually existed before already, because concurrent completions might have
resulted in an undefined state (e.g. concatenation of parts while the directory
is deleted concurrently, which is neither atomic nor isolated).