Transcoder lookup is inefficient

EncoderRegistryImpl stores transcoders in a set, without any lookup optimization because it assumes the number of transcoders is small.

However, InternalCacheFactory.bootstrap() registers a "different" TranscoderMarshallerAdapter for each cache, even though they all delegate to the same global marshaller.

I believe the global marshaller transcoder adapter should be registered only once in EncoderRegistryFactory, and ideally we should have a way of looking up transcoders by media types.