Optimize single-node CuratorCache storage with AtomicReference

[waterWang]

Summary

When a CuratorCache is built with the SINGLE_NODE_CACHE option, only one node is ever cached (the root path). Today the cache still uses a ConcurrentHashMap-backed StandardCuratorCacheStorage, which is overkill and wastes memory for this case. This PR adds a dedicated SingleNodeCuratorCacheStorage implementation backed by a single AtomicReference and switches the default storage selection to it whenever SINGLE_NODE_CACHE is set.

Fixes #1304

Changes

• New SingleNodeCuratorCacheStorage — implements CuratorCacheStorage with a single AtomicReference<ChildData>. All interface methods (put, remove, get, size, stream, clear) behave identically to StandardCuratorCacheStorage for the single-node case:
  • size() returns 0 or 1
  • stream() yields at most one element
  • put/get/remove match on the stored node's path, so a single-node cache can never hold entries for multiple paths.
  • Honors the cacheBytes flag the same way StandardCuratorCacheStorage does.
• CuratorCacheStorage.java — adds two factory methods: singleNode() and singleNodeDataNotCached().
• CuratorCacheImpl.java — the constructor now selects CuratorCacheStorage.singleNode() (instead of standard()) when SINGLE_NODE_CACHE is set and no explicit storage was provided by the caller. Calling builder(client, path).withStorage(...) still takes precedence.

Behavioral compatibility

• CuratorCacheStorage is a public interface and the storage is only switched when the caller did not supply a custom storage. Explicit withStorage() calls are unchanged.
• The SINGLE_NODE_CACHE option previously only controlled watcher recursion (recursive = !options.contains(SINGLE_NODE_CACHE)); this PR additionally optimizes the backing store without changing any observable semantics — get, size, stream, and events all behave the same.

Test plan

Existing TestCuratorCache (incl. testClearOnClose, testGreaterThan64kZNodes) and related cache tests cover the standard path and should remain green. Single-node behavior is exercised by tests that build caches with SINGLE_NODE_CACHE and verify get/size/stream semantics.