* Move transport logging into a new transport class TransportTraceDecorator (trace+ to get it from a url). * Give Registry a useful __repr__. * Fix a bug introduced by the change to use a Registry for transport where the transport loading tests left global state behind due to the _get_protocol_handlers method returning a reference, not a value object. * Add an upper_limit parameter to readv, because when asking for byte ranges within the latency-adjustment window near the end of the file causes errors that are tricky to manage. * A few minor drive-by formatting fixes. * The TransportDecorator constructor now accepts a _from_transport parameter for decorators that want to share state (used by the trace decorator)