Fix to set future done after the child is created

NioClientSocketTransport can't write data when childTransport is not
instantiated. f connect operation is done, then the childTransport
must be instantiated.
Fix timing on setting exception to future

IOException is thrown by NioChildSocketTransport because the IOException
is handled in ConnectSelector
Add settin point of WriteQueue

To use custoe WriteQueue, set its factory instance to
NioServer/ClientSocketConfig. The default is SipmpleWriteQueue.
The other implementation, DeficitRoundRobinWriteQueue is also added.
Remove MessageEvent

MessageEvent is redundant to pass a message between stages.
Transport is accessible via StageContext.