[#2664] Support write-spinning also in native transport

Motivation:

In our nio implementation we use write-spinning for maximize throughput, but in the native implementation this is not used.

Modification:

Respect writeSpinCount in native transport.

Result:

Better throughput
 [#2647] Handle IOV_MAX in java code

Motivation:

The handling of IOV_MAX was done in JNI code base which makes stuff really complicated to maintain etc.

Modifications:

Move handling of IOV_MAX to java code to simplify stuff

Result:

Cleaner code.