Refactoring in preparation to unify I/O logic for all branches

Motivation:

While trying to merge our ChannelOutboundBuffer changes we've made last
week, I realized that we have quite a bit of conflicting changes at 4.1
and master. It was primarily because we added
ChannelOutboundBuffer.beforeAdd() and moved some logic there, such as
direct buffer conversion.

However, this is not possible with the changes we've made for 4.0. We
made ChannelOutboundBuffer final for example.

Maintaining multiple branch is already getting painful and having
different core will make it even worse, so I think we should keep the
differences between 4.0 and other branches minimal.

Modifications:

- Move ChannelOutboundBuffer.safeRelease() to ReferenceCountUtil
- Add ByteBufUtil.threadLocalBuffer()
  - Backported from ThreadLocalPooledDirectByteBuf
- Make most methods in AbstractUnsafe final
  - Add AbstractChannel.filterOutboundMessage() so that a transport can
    convert a message to another (e.g. heap -> off-heap), and also
    reject unsupported messages
  - Move all direct buffer conversions to filterOutboundMessage()
  - Move all type checks to filterOutboundMessage()
- Move AbstractChannel.checkEOF() to OioByteStreamChannel, because it's
  the only place it is used at all
- Remove ChannelOutboundBuffer.current(Object), because it's not used
  anymore
- Add protected direct buffer conversion methods to AbstractNioChannel
  and AbstractEpollChannel so that they can be used by their subtypes
- Update all transport implementations according to the changes above

Result:

- The missing extension point in 4.0 has been added.
  - AbstractChannel.filterOutboundMessage()
  - Thanks to the new extension point, we moved all transport-specific
    logic from ChannelOutboundBuffer to each transport implementation
- We can copy most of the transport implementations in 4.0 to 4.1 and
  master now, so that we have much less merge conflict when we modify
  the core.