fix: concurrency groundwork for the multi-user client and features repository

[madhuchavva]

Concurrency groundwork for the multi-user client and features repository

Summary

• Fixes for thread-safety bugs
• adds a compile-time guard against post-Java-8 API usage.
• This is the groundwork PR for the async client work. No API changes.

What's included

• InMemoryStickyBucketServiceImpl — new no-arg constructor backed by ConcurrentHashMap, used by Options.setInMemoryStickyBucketService(). The previous plain HashMap is a structural data race (not just a lost update) when one client's concurrent evaluations save assignments in parallel. The map-injecting constructor is unchanged; its Javadoc now states the thread-safety requirement.
• GrowthBookClient — assigned is now a ConcurrentHashMap and callbacks a CopyOnWriteArrayList (both were plain collections mutated per run()/subscribe()). The assigned-variation change-check and publish in fireSubscriptions are now one atomic compute step, so two concurrent run() calls can no longer both observe a stale value and double-fire subscriptions; callbacks still run outside the map lock.
• GBFeaturesRepository callback dispatch — refreshCallbacks was a plain ArrayList: adds were synchronized but the dispatch iteration on the poll/SSE/retry background threads was not, so registering a callback during a refresh could throw ConcurrentModificationException. Now CopyOnWriteArrayList (the fix NativeJavaGbFeatureRepository already uses).
• GBFeaturesRepository polling thread — the SWR poll scheduler used the default (non-daemon) thread factory: the only non-daemon thread in the SDK, and it keeps the JVM alive if an application exits without calling shutdown(). Now a named daemon thread (growthbook-feature-poll), matching the SSE retry factory.
• FeatureSnapshot — featuresJson/savedGroupsJson/parsedFeatures/parsedSavedGroups were four independently assigned volatile fields, read separately by GrowthBookClient.buildGlobalContext, so a refresh landing between two reads produced a context with new features + old saved groups. They are now captured together in one immutable FeatureSnapshot swapped through a single AtomicReference; existing getters delegate, and buildGlobalContext reads the snapshot once. New public getFeatureSnapshot().

What's NOT changed:

• Legacy single-user GrowthBook internals (single-threaded by design; ).