Add type parameter
Fix comments
Add data output type parameter for state handler

The state handler, load(StageContext, TransportStateEvent) and
store(StageContext, TransportStateEvent) can output buffered data.
Call executeStore() on close operation if necessary

Give the store pipeline a chance to close resources when suddenly
the base socket is closed.
Proceed TransportStateEvent automatically

StageContext#proceed(TransportStaeEvent) is removed. For each
active load and store stages, load(StageContext, TransportStateEvent)
and store(StageContext, TransportStateEvent) is called.
Merge mock object