fix: Log the isFlagKnown cached-data warning only once per client

[kinyoklion]

Summary

While the client is not initialized but the data store already holds data, isFlagKnown logs a warning on every call. A client that stays in that state, for example one serving persisted data before its data source connects, emits that warning for every check, which floods the log without adding information.

The warning now logs once per LDClient instance. The check-and-set uses an AtomicBoolean.compareAndSet, which is lock-free and only runs on the path where the warning may be needed, so normal calls are unaffected. The message says that it is logged once.

The sibling message for an uninitialized store, and the gating condition itself, are unchanged. The evaluation path in this SDK has no equivalent cached-data message, so isFlagKnown is the only site.