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
Add `assert`?