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