[#2349] Correctly handle cancelled ChannelPromise in DefaultChannelHandlerContext

Motivation:
At the moment an IllegalArgumentException will be thrown if a ChannelPromise is cancelled while propagate through the ChannelPipeline. This is not correct, we should just stop to propagate it as it is valid to cancel at any time.

Modifications:
Stop propagate the operation through the ChannelPipeline once a ChannelPromise is cancelled.

Result:
No more IllegalArgumentException when cancel a ChannelPromise while moving through the ChannelPipeline.