Implement user-defined writability flags

Related: #2945

Motivation:

Some special handlers such as TrafficShapingHandler need to override the
writability of a Channel to throttle the outbound traffic.

Modifications:

Add a new indexed property called 'user-defined writability flag' to
ChannelOutboundBuffer so that a handler can override the writability of
a Channel easily.

Result:

A handler can override the writability of a Channel using an unsafe API.
For example:

  Channel ch = ...;
  ch.unsafe().outboundBuffer().setUserDefinedWritability(1, false);

For more information, take a look at the test*Writability() methods in ChannelOutboundBufferTest.