In-file metadata fix for SharedWriter with multiple streams
SharedWriter combines in-file metadata for all streams. To preserve objects with different content, MetaDataSvc will modify their keys so they become unique and not clash with each other.
The key modification inserts stream name into the key. Later on AthenaOutputStream will extract that part and distribute objects to correct streams.
This change is completely transparent (except to metadata copying tools).
This MR only enables the new behavior for FileMetaData objects - and serves as a bugfix.
Edited by Peter Van Gemmeren