Updated the ChannelGroup JavaDoc by removing b.releaseExternalResources();
[#2705] Call fireChannelReadComplete() if channelActive(...) decodes messages in ReplayingDecoder / ByteToMessageDecoder

Motivation:

In ReplayingDecoder / ByteToMessageDecoder channelInactive(...) method we try to decode a last time and fire all decoded messages throw the pipeline before call ctx.fireChannelInactive(...). To keep the correct order of events we also need to call ctx.fireChannelReadComplete() if we read anything.

Modifications:

- Channel channelInactive(...) to call ctx.fireChannelReadComplete() if something was decoded
- Move out.recycle() to finally block

Result:

Correct order of events.
[#2662] Fix race in cancellation of TimerTasks which could let to NPE

Motivation:

Due some race-condition while handling canellation of TimerTasks it was possibleto corrupt the linked-list structure that is represent by HashedWheelBucket and so produce a NPE.

Modification:

Fix the problem by adding another MpscLinkedQueue which holds the cancellation tasks and process them on each tick. This allows to use no synchronization / locking at all while introduce a latency of max 1 tick before the TimerTask can be GC'ed.

Result:

No more NPE
[#2692] Allows notify ChannelFutureProgressListener on complete writes

Motivation:

We have some inconsistency when handling writes. Sometimes we call ChannelOutboundBuffer.progress(...) also for complete writes and sometimes not. We should call it always.

Modifications:

Correctly call ChannelOuboundBuffer.progress(...) for complete and incomplete writes.

Result:

Consistent behavior
Use correct exception message when throw exception from native code

Motivation:

We sometimes not use the correct exception message when throw it from the native code.

Modifications:

Fixed the message.

Result:

Correct message in exception
[#2693] Reduce memory usage of ChannelOutboundBuffer

Motiviation:

ChannelOuboundBuffer uses often too much memory. This is especially a problem if you want to serve a lot of connections. This is due the fact that it uses 2 arrays internally. One if used as a circular buffer and store the Entries that are never released  (ChannelOutboundBuffer is pooled) and one is used to hold the ByteBuffers that are used for gathering writes.

Modifications:

Rewrite ChannelOutboundBuffer to remove these two arrays by:
  - Make Entry recyclable and use it as linked Node
  - Remove the circular buffer which was used for the Entries as we use a Linked-List like structure now
  - Remove the array that did hold the ByteBuffers and replace it by an ByteBuffer array that is hold by a FastThreadLocal. We use a fixed capacity of 1024 here which is fine as we share these anyway.
  - ChannelOuboundBuffer is not recyclable anymore as it is now a "light-weight" object. We recycle the internally used Entries instead.

Result:

Less memory footprint and resource usage. Performance seems to be a bit better but most likely as we not need to expand any arrays anymore.

Benchmark before change:
[nmaurer@xxx]~% wrk/wrk -H 'Host: localhost' -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' -H 'Connection: keep-alive' -d 120 -c 256 -t 16 --pipeline 256  http://xxx:8080/plaintext
Running 2m test @ http://xxx:8080/plaintext
  16 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    26.88ms   67.47ms   1.26s    97.97%
    Req/Sec   191.81k    28.22k  255.63k    83.86%
  364806639 requests in 2.00m, 48.92GB read
Requests/sec: 3040101.23
Transfer/sec:    417.49MB

Benchmark after change:

[nmaurer@xxx]~% wrk/wrk -H 'Host: localhost' -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' -H 'Connection: keep-alive' -d 120 -c 256 -t 16 --pipeline 256  http://xxx:8080/plaintext
Running 2m test @ http://xxx:8080/plaintext
  16 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    22.22ms   17.22ms 301.77ms   90.13%
    Req/Sec   194.98k    41.98k  328.38k    70.50%
  371816023 requests in 2.00m, 49.86GB read
Requests/sec: 3098461.44
Transfer/sec:    425.51MB