Optimize gathering write in the epoll transport

Motivation:

While benchmarking the native transport, I noticed that gathering write
is not as fast as expected.  It was due to the fact that we have to do a
lot of array copies to put the buffer addresses into the iovec struct
array.

Modifications:

Introduce a new class called IovArray, which allows to fill buffers
directly into an off-heap array of iovec structs, so that it can be
passed over to JNI without any extra array copies.

Result:

Big performance improvement when doing gathering writes:

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

After:

[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
Overall cleanup

- ChannelOutboundBuffer.Entry.buffers -> bufs for consistency
- Make Native.IOV_MAX final because it's a constant
- Naming changes
  - FlushedMessageProcessor -> MessageProcessor just in case we can
    reuse it for unflushed messages in the future
- Add ChannelOutboundBuffer.Entry.recycle() that does not return the
  next entry, and use it wherever possible
- Javadoc clean-up