Motivation:

In a proxy application it is desirable to propagate a user event
backward, from proxy destination end-point to client end-point.
Currently the only way is to set a List<ChannelHandler> attribute on
channel context and pass event object to them however benefits of
Netty's event firing (easy access to proper context, exception
handling, ...) is lost.

Modifications:

Very similar to fireUserEventTriggered() and userEventTriggered(), two
method fireUserEventTriggeredOutbound() and userEventTriggeredOutbound()
were added. in the channel handler context implementation instead of
next handler, previous handler is found and called.

Result:

It's possible to send a user event from end of pipeline to the beginning
of it. However the changes are backward-incompatible and direct implementations
 of ChannelHandler and ChannelHandlerContext need to implement the
methods.

Rename userEventTriggeredOutbound to userEventTriggeredBackward.
Make event invoker compatible with the new event userEventTriggeredBackward
(previously called userEventTriggeredOutbound).

Modifications:

Added the proper methods to Invoker interface and it's implementations.

Result:

It's possible to send an event from end of pipeline to beginning of it
and capture it via userEventTriggeredBackward() in channel handlers.