ewan higgs <> in hadoop

HADOOP-16193. Add extra S3A MPU test to see what happens if a file is created during the MPU. Contributed by Steve Loughran

HDFS-13713. Add specification of Multipart Upload API to FS specification, with contract tests.

Contributed by Ewan Higgs and Steve Loughran.

HDFS-13936. Multipart upload to HDFS to support 0 byte upload. Contributed by Ewan Higgs.