IoUringAssmEngn: Implement shouldAcceptRequests daemon/async control

We've reworked the synchronous control functions that govern the
async daemon and in-flight requests for this class. The
shouldAcceptRequests flag represents the readiness state of the
whole engine class. The in-flight async operations consult the
shouldAcceptRequests flag to determine whether they should return
early.

Now the stop() method is solely for setting the locked flag
shouldAcceptRequests=false.

The pair resetAndAssembleFrame()/assemblyCycleComplete manage the
per-assembly cycle state machine, and they don't need to set or
interfere with the shouldAcceptRequests flag.

stimBuffApis/livoxGen1/ioUringAssemblyEngine.h

@@ -47,9 +47,10 @@ public:
 		{ return nSucceeded != 0 && nTotal != 0 && nSucceeded != nTotal; }
 
 private:
-	typedef std::function<void(void*, int)> resetAndAssembleFrameCbFn;
+	typedef std::function<void(void*, int, bool&)> resetAndAssembleFrameCbFn;
 	void resetAndAssembleFrame(resetAndAssembleFrameCbFn onCqeReady);
-	void stop(bool doAcquireLock = true);
+	void assemblyCycleComplete();
+	bool stop();
 
 private:
 	PcloudStimulusBuffer& parent;
@@ -59,7 +60,6 @@ private:
 
 	// io_uring infrastructure
 	struct io_uring ring;
-	bool isSetup;
 
 	// Eventfd for CQE notifications (used with boost's unified loop)
 	int eventfdFd;
@@ -72,12 +72,13 @@ private:
 	boost::asio::deadline_timer stallTimer;
 	// Callback for CQE ntfns (called with user_data+result from each CQE)
 	resetAndAssembleFrameCbFn onCqeReadyCallback;
-	// Flag to indicate assembly is in progress (cleared by stop())
-	// Protected by isAssemblingLock
-	SpinLock isAssemblingLock;
-	bool isAssembling;
+	/**	EXPLANATION:
+	 * Flag to indicate whether engine should accept new requests.
+	 * Set by setup(), cleared by stop().
+	 */
+	SpinLock shouldAcceptRequestsLock;
+	bool shouldAcceptRequests;
 
-	// Number of datagrams per staging buffer frame
 	size_t nDgramsPerStagingBufferFrame;
 
 	struct SlotAssemblyDesc