The Interlocked.Exchange calls will ensure that the same object is
returned from each call even under a race.  That's not necessarily
required, but it should also make sure that no write re-ordering issues
occur on platforms that allow it.  And it also makes the possibility
of thread safety issues explicit.

Intialize it to null to match pattern for the other references.
Fixes the case were the lazy initialization would break when
public_key_token was Empty<byte>.Array.