IDokanOperations2 library implementation

[LTRData]

Hi all. I just published a new branch:
https://github.com/dokan-dev/dokan-dotnet/tree/IDokanOperations2

This is a modernization of the library in several ways, but most notably a new interface IDokanOperations2 that is a more modern approach for implementing Dokan file systems in .NET with less CPU and memory pressure. The old IDokanOperations interface still exists and is implemented using a compatibility layer that in turn implements the new IDokanOperations2 interface.

This is a way to start working towards merging in changes I did in my fork at https://github.com/LTRData/dokan-dotnet.

Most important changes:

• IDokanOperations2 is a new way to implement Dokan file systems in .NET with less CPU and memory pressure.
• Includes several other minor optimizations as well.
• Existing IDokanOperation and IDokanOperationUnsafe implementations get wrapped in a compatibility layer called DokanOperationsAdapter, that in turn implements the new IDokanOperations2 interface.
• Removed "-windows" suffix from target frameworks to allow library to be consumed by applicaitons that chose between Windows and other OS implementations at runtime.
• Removed .NET Framework 4.0 from target frameworks and added 4.6 and 4.8 (last supported on Windows Vista and last supported on Windows 7).
• DokanNetMirror with IDokanOperations2. Old DokanNetMirror moved to DokanNetMirrorLegacy.
• Added some logic to keep a reference to DokanInstance and associated operations alive while file system is mounted, to avoid crashes in native code if the last reference is collected by GC.

Now, even though the code has been used for a while through separate packages that I built, this should still be considered "work-in-progress" until we are confident with code struture, namespaces, class names etc that I am sure people could have valuable suggestions about.