Make OdbBackend.Read[Prefix]() methods return an UnmanagedMemoryStream

Currently those methods return an out `Stream`. However, the underlying code **[expects](https://github.com/libgit2/libgit2sharp/blob/vNext/LibGit2Sharp/OdbBackend.cs#L247)** it to be an `UnmanagedMemoryStream`.

Let's make this explicit through a change in the parameter type rather than silently failing.