Fix type parameters at LoadStage/StoreStage

Type parameter I and O is not concerned abount TransportStateEvent.
So change <I, O> to <?, ?> at LoadStage/StoreStage#execute(StageContext,
TransportEvent).
Remove package niotty.codec.string
Add factory method to create CodecBufferList

The method creates CodecBufferList with a priority.
Fix the type of @SupressWarnings