[#2752] Add PendingWriteQueue for queue up writes

Motivation:

Sometimes ChannelHandler need to queue writes to some point and then process these. We currently have no datastructure for this so the user will use an Queue or something like this. The problem is with this Channel.isWritable() will not work as expected and so the user risk to write to fast. That's exactly what happened in our SslHandler. For this purpose we need to add a special datastructure which will also take care of update the Channel and so be sure that Channel.isWritable() works as expected.

Modifications:

- Add PendingWriteQueue which can be used for this purpose
- Make use of PendingWriteQueue in SslHandler

Result:

It is now possible to queue writes in a ChannelHandler and still have Channel.isWritable() working as expected. This also fixes #2752.
Shouldn't this be inlined and eliminated?  If you wannt keep it this way, you should rename the method because it does no assertion by itself.