Separate unmanaged pointer lifecycle management into SafeHandles

[jmaeagle99]

#583 introduced separate SafeHandle implementations for the Client and Worker bridge implementations to allow the SafeHandles to be passed around and prevent use-after-free problems. This should be extended to the remaining classes that currently maintain the lifetime of unmanaged pointers.

• Pointer lifetime management should be exclusively maintained by SafeHandle types for each unmanaged pointer type.
• Consumers of the unmanaged pointers should use the Scope.Pointer method to add-ref on a SafeHandle, which will return the unmanaged pointer value.