orb: add a separate uORB::Manager::initialize() method

This fixes a race condition: uORB::Manager::get_instance() is used in a
multi-thread context, but the singleton initialization was not thread-safe.
Further, this avoids having to check for nullptr every time the singleton
is accessed.

uORB::Manager::initialize() is called when uorb is started. No one else
accesses the singleton before that point, because it is only used in the
orb_* methods, and in muorb. Both require uorb to be started already when
they are used.

src/modules/uORB/uORBMain.cpp

@@ -33,6 +33,7 @@
 
 #include <string.h>
 #include "uORBDevices.hpp"
+#include "uORBManager.hpp"
 #include "uORB.h"
 #include "uORBCommon.hpp"
 
@@ -70,6 +71,11 @@ uorb_main(int argc, char *argv[])
 			return 0;
 		}
 
+		if (!uORB::Manager::initialize()) {
+			PX4_ERR("uorb manager alloc failed");
+			return -ENOMEM;
+		}
+
 		/* create the driver */
 		g_dev = new uORB::DeviceMaster(uORB::PUBSUB);