AbstractEventStream: use copy-on-write list implementation

[moio]

I verified in Uyuni that the previous implementation might lead to excessive monitor contention over the list, especially since iteration (in notifyListeners) requires a monitor.

In the application that I am looking at, we call notifyListeners from multiple threads, and this results in all threads but one being blocked while the notification is in progress.

This change uses a different synchronized Collection which has an extra cost on writes but never blocks reads.

[lucidd]

if i remember correctly the new ArrayList here was done in case the callback of a listener modifies the listener list resulting in a ConcurentModificationException. From the CopyOnWriteArray docs it seems that it does not have this problem.

... and the iterator isguaranteed not to throw {@code ConcurrentModificationException}

[moio]

Right, fixing.