new IntrusiveSortedList container used for uORB, WorkQueues, and WorkItems

- new intrusive linked list container (c++ template) that sorts on insertion
 - primarily for convenience inspecting things in the system like uORB or WorkQueues
 - uorb status or top sorted alphabetically 
 - work_queue status threads sorted by priority, then items sorted alphabetically within each

src/modules/uORB/uORBDeviceMaster.hpp

@@ -50,7 +50,7 @@ class Manager;
 #include <string.h>
 #include <stdlib.h>
 
-#include <containers/List.hpp>
+#include <containers/IntrusiveSortedList.hpp>
 #include <px4_platform_common/atomic_bitset.h>
 
 using px4::AtomicBitset;
@@ -85,7 +85,7 @@ public:
 	 * Continuously print statistics, like the unix top command for processes.
 	 * Exited when the user presses the enter key.
 	 * @param topic_filter list of topic filters: if set, each string can be a substring for topics to match.
-	 *        Or it can be '-a', which means to print all topics instead of only currently publishing ones.
+	 *        Or it can be '-a', which means to print all topics instead of only ones currently publishing with subscribers.
 	 * @param num_filters
 	 */
 	void showTop(char **topic_filter, int num_filters);
@@ -114,7 +114,7 @@ private:
 	 */
 	uORB::DeviceNode *getDeviceNodeLocked(const struct orb_metadata *meta, const uint8_t instance);
 
-	List<uORB::DeviceNode *> _node_list;
+	IntrusiveSortedList<uORB::DeviceNode *> _node_list;
 	AtomicBitset<ORB_TOPICS_COUNT> _node_exists[ORB_MULTI_MAX_INSTANCES];
 
 	px4_sem_t	_lock; /**< lock to protect access to all class members (also for derived classes) */

src/modules/uORB/uORBDeviceNode.hpp

@@ -38,6 +38,7 @@
 
 #include <lib/cdev/CDev.hpp>
 
+#include <containers/IntrusiveSortedList.hpp>
 #include <containers/List.hpp>
 #include <px4_platform_common/atomic.h>
 
@@ -52,7 +53,7 @@ class SubscriptionCallback;
 /**
  * Per-object device instance.
  */
-class uORB::DeviceNode : public cdev::CDev, public ListNode<uORB::DeviceNode *>
+class uORB::DeviceNode : public cdev::CDev, public IntrusiveSortedListNode<uORB::DeviceNode *>
 {
 public:
 	DeviceNode(const struct orb_metadata *meta, const uint8_t instance, const char *path, ORB_PRIO priority,
@@ -65,6 +66,8 @@ public:
 	DeviceNode(DeviceNode &&) = delete;
 	DeviceNode &operator=(DeviceNode &&) = delete;
 
+	bool operator<=(const DeviceNode &rhs) const { return (strcmp(get_devname(), rhs.get_devname()) <= 0); }
+
 	/**
 	 * Method to create a subscriber instance and return the struct
 	 * pointing to the subscriber as a file pointer.