To avoid viewers (particularly on the Hypergrid) from permanently caching a UUID -> "Unknown User" binding, drop the binding request rather than replying with "Unknown User"

By not binding UUID -> "Unknown User", we leave open the possibility that the binding may be correctly resolved at a later time, which can still happen in some Hypergrid situations.
Observed behaviour of LL viewer 3.3.4 is that a dropped bind request is not retried until the next session.