Issue #5108 - improve scalability of WebSocket SessionTrackers

[lachlan-roberts]

Issue #5108

Use a set based on ConcurrentHashMap instead of CopyOnWriteArraySet for the WebSocket SessionTrackers.

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The current data structure for SessionTracker is CopyOnWriteArraySet, which has the following description in the javadoc.

• It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
• Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array.
• Traversal via iterators is fast and cannot encounter interference from other threads.

This seems particularly badly suited to our use case.

• We probably will not have a small set size.
• Write operations usually outnumber the read operations. (assuming they don't call getSessions() very often).
• It may be the case where we never iterate over the open sessions until doStop().