NR-586737 changed reachability to update a callback on change instead of calling it every time

[mbruin-NR]

1. _Atomic(int) _cachedStatus ivar — lock-free cache for the last known status. int rather than the enum directly to avoid any C11 _Atomic-on-enum edge cases.
2. NRMAReachabilityCallback + setup in +reachability — one synchronous SCNetworkReachabilityGetFlags call at creation time primes the cache, thenSCNetworkReachabilitySetDispatchQueue hands off all future updates to a utility queue. After this, no thread ever blocks on reachability.
3. dealloc — cancels the callback and queue before CFRelease. This is the critical safety pairing:text (non-owning) is safe only because dealloc guarantees no callback fires after the object is freed.
4. CurrentReachabilityStatus that checkOfflineStatus called on every event no longer touches the Network framework.