Improve SessionTracker scalability

[sbordet]

Jetty version
9.4.26 and later

Description
SessionTracker holds a list of session in a COWAList.

Unfortunately it's a singleton-like object, so if an application opens and closes WebSocket connections at a high rate, it becomes a bottleneck because of lock contention.

[joakime]

IIRC, The SessionTracker has a few purposes.

1. To avoid the addBean() removeBean() contention of the previous impl (and dump() issues)
2. Holds the collection of open Sessions to allow for proper graceful shutdown.
3. To satisfy the needs of javax.websocket.Session.getOpenSessions()

While 1 and 2 can be solved in ways that don't need to use the Collections objects, the need in point 3 means the the ability to at least return a Set<Session> when asked.

[sbordet]

@joakime do we only need it for the JSR implementation, or also the Jetty APIs have a similar method?

We can use a more efficient data structure as getOpenSessions() is not called that often, so that we do not pay the price every time a connection is opened/closed.

[lachlan-roberts]

@sbordet the Jetty WebSocket API does have similar methods as well.

[lachlan-roberts]

PR #5109 fixes the scalability issue by using a set derived from ConcurrentHashMap instead of the CopyOnWriteArraySet.
This has been merged into the 9.4.x branch and will be available in a 9.4.32 release.