Port ChannelOutboundBuffer and related changes from 4.0

Motivation:

We did various changes related to the ChannelOutboundBuffer in 4.0 branch. This commit port all of them over and so make sure our branches are synced in terms of these changes.

Related to [#2734], [#2709], [#2729], [#2710] and [#2693] .

Modification:
Port all changes that was done on the ChannelOutboundBuffer.

This includes the port of the following commits:
 - 73dfd7c01b49aca006a34cc48197dee3fc360af1
 - 997d8c32d23f2d88903b7b607360907b99101002
 - e282e504f17b0874719ff606c728494e3509b1a0
 - 5e5d1a58fd3159c04ac7d10edfb8ed7a83d3935e
 - 8ee3575e72d6ee000a99c717d96f36695a8667a0
 - d6f0d12a8692c095df43b2a4462cbc97cf5c5a2d
 - 16e50765d1fb99005ad761409c28dcedf477531b
 - 3f3e66c31ae3da70c36cc125ca9bcac8215390e4

Result:
 - Less memory usage by ChannelOutboundBuffer
 - Same code as in 4.0 branch
 - Make it possible to use ChannelOutboundBuffer with Channel implementation that not extends AbstractChannel