orb: rm static from DeviceMaster::_node_map & use the non-static getDeviceNode in uORB::Manager

Reasons:
- DeviceMaster::_node_map does not need to be shared among instances,
  because there is at most 1 instance per Flavor and different Flavors
  have non-intersecting device paths.
- Keeping it static would also require a static lock
- DeviceMaster::_node_map was not locked at all when used from
  uORB::Manager

So this fixes two synchronization issues:
- Different DeviceMaster objects could access the same static data in
  parallel
- getDeviceNode() called from uORB::Manager did not use any locking at all

src/modules/uORB/uORBDevices_posix.cpp

@@ -44,8 +44,6 @@
 #include <px4_sem.hpp>
 #include <stdlib.h>
 
-std::map<std::string, uORB::DeviceNode *> uORB::DeviceMaster::_node_map;
-
 
 uORB::DeviceNode::SubscriberData  *uORB::DeviceNode::filp_to_sd(device::file_t *filp)
 {
@@ -742,7 +740,7 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 					if (ret == -EEXIST) {
 						/* if the node exists already, get the existing one and check if
 						 * something has been published yet. */
-						uORB::DeviceNode *existing_node = GetDeviceNode(devpath);
+						uORB::DeviceNode *existing_node = getDeviceNodeLocked(devpath);
 
 						if ((existing_node != nullptr) && !(existing_node->is_published())) {
 							/* nothing has been published yet, lets claim it */
@@ -779,7 +777,17 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 	}
 }
 
-uORB::DeviceNode *uORB::DeviceMaster::GetDeviceNode(const char *nodepath)
+uORB::DeviceNode *uORB::DeviceMaster::getDeviceNode(const char *nodepath)
+{
+	lock();
+	uORB::DeviceNode *node = getDeviceNodeLocked(nodepath);
+	unlock();
+	//We can safely return the node that can be used by any thread, because
+	//a DeviceNode never gets deleted.
+	return node;
+}
+
+uORB::DeviceNode *uORB::DeviceMaster::getDeviceNodeLocked(const char *nodepath)
 {
 	uORB::DeviceNode *rc = nullptr;
 	std::string np(nodepath);