ConcurrentModification as IOConnection.register() does not take a lock

[EugenDueck]

The for-loop
for (IOConnection connection : list)
in that method throws ConcurrentModificationExceptions, when I create hundreds of client instances (for benchmarking purposes).

I haven't yet familiarized myself with the codebase, but when I just put a
synchronized (connections)
around the full body of that method (which is maybe too coarse) it worked fine.

FYI: In cleanup(), you already synchronize on the connections object in order to access it

[Gottox]

Thanks, I applied your proposal.