libpbd: provide a way to abort an RCU-based write_copy()->update() sequence

libs/pbd/pbd/rcu.h

@@ -184,11 +184,15 @@ public:
 
 		return new_copy;
 
-		/* notice that the write lock is still held: update() MUST
+		/* notice that the write lock is still held: update() or abort() MUST
 		 * be called or we will cause another writer to stall.
 		 */
 	}
 
+	void abort () {
+		_lock.unlock();
+	}
+
 	bool update (boost::shared_ptr<T> new_value)
 	{
 		/* we still hold the write lock - other writers are locked out */