Remove duplicate range check in AbstractByteBuf.skipBytes()
Fix a regression caused by 73dfd7c01b49aca006a34cc48197dee3fc360af1

Motivation:

73dfd7c01b49aca006a34cc48197dee3fc360af1 introduced various test
failures because:

- EpollSocketChannel.doWrite() raised a NullPointerException when
  notifying the write progress.
- ChannelOutboundBuffer.nioBuffers() did not expand the internal array
  when the pending entries contained more than 1024 buffers, dropping
  the remainder.

Modifications:

- Fix the NPE in EpollSocketChannel by removing an unnecessary progress
  update
- Expand the thread-local buffer array if there is not enough room,
  which was the original behavior dropped by the offending commit

Result:

Regression is gone.
Optimize native transport for gathering writes

Motivation:

While benchmarking the native transport with gathering writes I noticed that it is quite slow. This is due the fact that we need to do a lot of array copies to get the buffers into the iov array.

Modification:

Introduce a new class calles IovArray which allows to fill buffers directly in a iov array that can be passed over to JNI without any array copies. This gives a nice optimization in terms of speed when doing gathering writes.

Result:

Big performance improvement when doing gathering writes. See the included benchmark...

Before:
[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    23.44ms   16.37ms 259.57ms   91.77%
    Req/Sec   181.99k    31.69k  304.60k    78.12%
  346544071 requests in 2.00m, 46.48GB read
Requests/sec: 2887885.09
Transfer/sec:    396.59MB

With this 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    21.93ms   16.33ms 305.73ms   92.34%
    Req/Sec   194.56k    33.75k  309.33k    77.04%
  369617503 requests in 2.00m, 49.57GB read
Requests/sec: 3080169.65
Transfer/sec:    423.00MB
Simplify and so remove the need to have EpollChannelOutboundBuffer at all