Anoter pass at adding [NotNull] attributes
A few tweaks and misc refactoring
Used  [InstantHandle] attribute and other fixes to reduce false positives in Resharper
Fixed AsyncTaskBuffer<T> running in CompletionOrder sometimes deadlocking due to a subtile race condition

- seemed to come from a race condition when setting the m_completionLock mutex in async mode
- changed AsyncCancellableMutex to have its own completed flag
- added AsyncCancellableMutex.AlreadDone singleton to not have to check the mutex to null everytime
Added a few methods to IFdbSubspace

- Contains(Slice) returns true if the slice logically exists in the subspace (note: can be tricky for database that allow access to the system keyspace)
- subspace[Slice] and subspace[IFdbKey] indexers that create a sub partition
Cleaned up the FdbCounter and FdbStringIntern classes

- moved back transactional overrides into the classes themselves
Fixed potential "access to modified closure" warning
Cleanup, tweaks, and XML comments
Fixed some Resharper warnings
Fixed wrong computation of final slice size in Slice.Concat(params Slice[])
Added solution-wide Resharper setting file to disable some code analysis rules
Fixed typo in last refactoring
Preparing for nuget packages release: changing version number to 0.9.7

- NuGet likes semantic versionning, so we can't really use 0.200.x or 0.300.x