Rewrite bridge implementation in DefaultChannelHandlerContext

This commit splits bridge into two parts.  One is NextBridgeFeeder,
which provides ByteBuf and MessageBuf that are local to the context
whose next*Buffer() has been invoked on.  The other is a thread-safe
queue that stores the data fed by NextBridgeFeeder.feed().

By splitting the bridge into the two parts, the data pushed by a handler
is not lost anymore when the next handler who provided the next buffer
is removed from the pipeline.

- Fixes #1272