Introduce ConcurrentAtomic and fix data race condition issue in Cocoa…

[jeffery812]

#630

ThreadSanitizer reported a data race issue when accessing the CocoaMQTTWebSocket.delegate and CocoaMQTTLogger.minLevel properties. The existing implementation assigns these properties asynchronously using a serial queue:

internal var internalQueue = DispatchQueue(label: "CocoaMQTTWebSocket")

public func setDelegate(_ theDelegate: CocoaMQTTSocketDelegate?, delegateQueue: DispatchQueue?) {
    internalQueue.async {
        self.delegate = theDelegate
        self.delegateQueue = delegateQueue
    }
}

Although a serial queue is used, the asynchronous call (async) does not guarantee timing consistency, which could lead to concurrent reads and writes if other parts of the code access delegate outside this queue.

Solution

Introduce ConcurrentAtomic object wrapper and annotate CocoaMQTTWebSocket.delegate and CocoaMQTTLogger.minLevel, make them thread safe. This ensures:
Multiple concurrent reads are allowed.
Writes are serialized with a barrier and will not happen concurrently with reads or other writes.

This approach ensures thread-safe reads and writes without blocking non-mutating operations.