gossip: investigate if neighbors list would benefit from a CopyOnWriteArrayList-like

[thibault-martinez]

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i.e. removing neighbors rw_lock https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CopyOnWriteArrayList.html https://stackoverflow.com/questions/17853112/in-what-situations-is-the-copyonwritearraylist-suitable