Allow to use ChannelOutboundBuffer without AbstractChannel

Motivation:

We expose ChannelOutboundBuffer in Channel.Unsafe but it is not possible
to create a new ChannelOutboundBuffer without an AbstractChannel.  This
makes it impossible to write a Channel implementation that does not
extend AbstractChannel.

Modifications:

- Change ChannelOutboundBuffer to take a Channel as constructor argument.
- Add javadocs

Result:

ChannelOutboundBuffer can be used with a Channel implemention that does
not extend AbstractChannel.