Deleting redundant needsFlush boolean

Motivation:

In ChunkedWriteHandler, there is a redundant variable that servers
no purpose. It implies that under some conditions you might not want
to flush.

Modifications:

Removed the variable and the if condition that read it. The boolean
was always true so just removing the if statement was fine.

Result:

Slightly less misleading code.
Adding origins whitelist support for CORS

Motivation:
Currently the CORS support only handles a single origin, or a wildcard
origin. This task should enhance Netty's CORS support to allow multiple
origins to be specified. Just being allowed to specify one origin is
particulary limiting when a site support both http and https for
example.

Modifications:
- Updated CorsConfig and its Builder to accept multiple origins.

Result:
Users are now able to configure multiple origins for CORS.

[https://github.com/netty/netty/issues/2346]
[#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.