fix: thread-safe cache writes and feature update handling

[vazarkevych]

Problem

Several race conditions existed in cache and feature update handling:

• InMemoryFeatureCache had no locking — concurrent reads/writes could corrupt cache entries
• FeatureRepository.load_features and load_features_async had no double-checked locking — multiple threads/coroutines could trigger redundant HTTP fetches simultaneously
• _feature_update_callbacks list was mutated and iterated without a lock — concurrent add/remove/notify could raise RuntimeError: list changed size during iteration
• _sticky_bucket_cache_lock was a boolean flag instead of a real lock — the spin-loop was not thread-safe and silently returned {} when the "lock" was held
• FeatureCache.get_current_state returned a mutable reference to savedGroups instead of a copy
• GrowthBook.load_features_async called save_in_cache with wrong cache key (client_key instead of api_host::client_key), making the cached value unreachable
• _features_event_handler had the same incorrect cache key

Changes

• InMemoryFeatureCache: added threading.Lock to get, set, clear
• FeatureRepository: added _load_lock and _async_load_lock with double-checked locking pattern in load_features and load_features_async
• _feature_update_callbacks: protected add/remove with _refresh_lock; _notify copies the list under the lock and iterates outside to prevent deadlocks from slow callbacks
• _sticky_bucket_cache_lock: replaced boolean spin-lock with asyncio.Lock(); simplified _refresh_sticky_buckets
• FeatureCache.get_current_state: returns dict() copy of savedGroups
• GrowthBook.load_features_async: removed redundant save_in_cache call (already handled by FeatureRepository)
• _features_event_handler: fixed cache key to api_host::client_key; changed return None to return