InMemory - Concurrency Issues

[4eyecoder]

Using InMemory during Unit and Integration Tests, we would get the following error:

System.IndexOutOfRangeException : Index was outside the bounds of the array.

at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
at Stowage.Impl.InMemoryFileStorage.Add(String path, DataStream sourceStream)
at System.IO.Stream.Close
at System.IO.Stream.DisposeAsync()

I have submitted a PR to resolve this issue by switching it to a ConcurrentDictionary

https://github.com/aloneguid/stowage/pull/25