Reacting to changes to MemoryManager (remove Length)

[ahsonkhan]

Part of https://github.com/dotnet/corefx/issues/28751

Depends on new corefx packages with this change: dotnet/corefx#28981

cc @davidfowl, @pakrym, @KrzysztofCwalina, @GrabYourPitchforks, @benaadams

[ahsonkhan]

Depends on new corefx packages with this change: dotnet/corefx#28981

Actually, given Length was a virtual property (not required to be overridden) and there is no one referencing MemoryPoolBlock.Length, we can merge this as is (not a source breaking change to aspnet).

MemoryPoolBlock (Debug) is an internal sealed class (and no one outside of the Common repo uses it anyway).

cc @joshfree

[benaadams]

return new Memory<byte>(this, 0, _length);

will also need to change to call the protected

return CreateMemory(_length);

As that public .ctor is now internal (due to its unsafe nature)

[ahsonkhan]

As that public .ctor is now internal (due to its unsafe nature)

Good point. Thanks for the reminder. We can still remove the Length property though. I will address this change separately. Can we merge this @pakrym?