Change TransportAggregate role

TransportAggregate shows that its implemented class aggregates
child Transports.
Make EncodeBufferGroup convenient to use.

EncodeBufferGroup implements BufferSink, and is added peek method.
Add Transport methods

add isOpen(), localAddress() and remoteAddress(). If the implemetation
of Transport is not support some of these, throws
UnsupportedOperationException.
Change PineLineFactory setting point.

To instantiate a class of TransportConfig without user-define
PipeLineFactory, the PipeLineFactory is set by setter.
Remove unnecessary comment.
Add packet frame length encoder/decoder

LengthPrependEncoder prepends contents size of byte before the
contents. LengthRemoveDecoder remove content length bytes prepended
by LengthPrependEncoder.

Fix ArrayDecodeBuffer boundary check for read primitives.
Fix the return type of remoteAddress()