Settings streams exchange semantics

[JohnMcPMS]

Fixes #463

Change

The goal of this change is to add exchange semantics to settings stream writes. Currently they do not have any interprocess synchronization. This does not lead to corrupted streams as the underlying systems prevent simultaneous writes, but it can lead to data loss if updates from one process are not merged with those from another.

Because holding any kind of lock would likely lead to contention amongst the processes, we use exchange semantics instead. Writes are only permitted if the stream has not changed since it was last read. This required moving to a more object oriented model for the settings streams, which then required updating the consumers of it to also maintain an object and respond to failed write attempts.

The update to AdminSettings was fairly straightforward, requiring just the necessary interface updates and the exchange retry loop.

For SourceListInternal, I also refactored the code out of RepositorySource.cpp which was getting quite large. I moved the group policy related functions to their own file (SourcePolicy.cpp) and the source enumeration related functions to theirs (SourceList.cpp). The renamed SourceList type now maintains the stream objects for its lifetime and manages the necessary exchange retry attempts for the various writes that are required.

Validation

New unit tests are added to validate the code under controlled circumstances. Functional tests would be impractical to write as the actual interactions are races between various writes, although the configurable test source that was added recently would aid in this area.

Microsoft Reviewers: Open in CodeFlow

[yao-msft]