Qutex: Add gridlock detection wireframing

This is the basic wireframing required to enable us to detect
gridlocks among requests.

include/serializedAsynchronousContinuation.h

@@ -99,13 +99,16 @@ public:
 
 			Qutex *firstFailedQutexPtr = nullptr;
 			bool deadlockLikely = isDeadlockLikely();
+			bool gridlockLikely = isGridlockLikely();
 
 			if (!serializedContinuation.requiredLocks.tryAcquireOrBackOff(
-				*this, (deadlockLikely ? &firstFailedQutexPtr : nullptr)))
+				*this,
+				((deadlockLikely || gridlockLikely)
+					? &firstFailedQutexPtr : nullptr)))
 			{
 				// Just allow this lockvoker to be dropped from its io_service.
 				allowAwakening();
-				if (!deadlockLikely)
+				if (!deadlockLikely && !gridlockLikely)
 					{ return; }
 
 #ifdef CONFIG_ENABLE_DEBUG_LOCKS
@@ -113,7 +116,10 @@ public:
 				bool isDeadlock = traceContinuationHistoryForDeadlockOn(
 					firstFailedQutex);
 
-				if (!isDeadlock)
+				bool isGridlock = traceContinuationHistoryForGridlockOn(
+					firstFailedQutex);
+
+				if (!isDeadlock && !isGridlock)
 					{ return; }
 
 				handleDeadlock(firstFailedQutex);
@@ -202,8 +208,13 @@ public:
 #endif
 		}
 
+		// Wrapper around isDeadlockLikely for gridlock detection
+		bool isGridlockLikely() const
+			{ return isDeadlockLikely(); }
+
 #ifdef CONFIG_ENABLE_DEBUG_LOCKS
 		bool traceContinuationHistoryForDeadlockOn(Qutex &firstFailedQutex);
+		bool traceContinuationHistoryForGridlockOn(Qutex &firstFailedQutex);
 		bool traceContinuationHistoryForDeadlock(void)
 		{
 			for (auto& lockUsageDesc

smocore/serializedAsynchronousContinuation.cpp

@@ -57,6 +57,17 @@ SerializedAsynchronousContinuation<OriginalCbFnT>
 	return false;
 }
 
+template <class OriginalCbFnT>
+template <class InvocationTargetT>
+bool
+SerializedAsynchronousContinuation<OriginalCbFnT>
+::LockerAndInvoker<InvocationTargetT>
+::traceContinuationHistoryForGridlockOn(Qutex &firstFailedQutex)
+{
+	// Empty implementation - to be filled in later
+	return false;
+}
+
 template <class OriginalCbFnT>
 template <class InvocationTargetT>
 void