Final fix

Act as in Netty 4/5 by merging interestOps and unwritable properties in
various AbstractChannel methods as:
- clearUserDefinedWritability and setUserDefinedWritability in
AbstractChannel to set interestOps as unwritable new status previously
set
- setWritable and setUnwritable used respectively in
WriteRequestQueue.poll and WriteRequestQueue.offer in AbstractNioChannel
to set unwritable status (global one) as interestOps new status (will be
set)
- getInterestOps in AbstractNioChannel to check unwritable user status
set before checking high and low WaterMark