[#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.
[#2666] Fix possible NPE when try to fullfill connect ChannelPromise

Motivation:

Because of a missing return statement we may produce a NPE when try to fullfill the connect ChannelPromise when it was fullfilled before.

Modification:

Add missing return statement.

Result:

No more NPE.
[#2667] Write until EAGAIN in native transport

Motivation:

We need to continue write until we hit EAGAIN to make sure we not see an starvation

Modification:

Write until EAGAIN is returned

Result:

No starvation when using native transport with ET.
[#2665] Continue writing on IOException when using DatagramChannel

Motivation:

As a DatagramChannel supports to write to multiple remote peers we must not close the Channel once a IOException accours as this error may be only valid for one remote peer.

Modification:

Continue writing on IOException.

Result:

DatagramChannel can be used even after an IOException accours during writing.
[#2667] Write until EAGAIN in native transport and only call setEpollOut() in this case

Motivation:

In the previous fix for #2667 I did introduce a bit overhead by calling setEpollOut() too often.

Modification:

Only call setEpollOut() if really needed and remove unused code.

Result:

Less overhead when saturate network.
Migrate Netty to use a ForkJoinPool by default
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