Allow ChunkedInput to provide the progress of its transfer

Related issue: #2741 and #2151

Motivation:

There is no way for ChunkedWriteHandler to know the progress of the
transfer of a ChannelInput. Therefore, ChannelProgressiveFutureListener
cannot get exact information about the progress of the transfer.

If you add a few methods that optionally provides the transfer progress
to ChannelInput, it becomes possible for ChunkedWriteHandler to notify
ChannelProgressiveFutureListeners.

If the input has no definite length, we can still use the progress so
far, and consider the length of the input as 'undefined'.

Modifications:

- Add ChunkedInput.progress() and ChunkedInput.length()
- Modify ChunkedWriteHandler to use progress() and length() to notify
  the transfer progress

Result:

ChunkedWriteHandler now notifies ChannelProgressiveFutureListener.