This fixes the case where a topic instance is already subscribed, and
advertised later. The subscriber will create the DeviceNode with default
priority, and the advertiser will just use the existing DeviceNode,
without updating to the requested priority.
Existing users of orb_exists:
- logger (dynamic subscribe to multi-instances)
- mavlink (orb subscription)
- sdlog2
- preflightcheck (check for home_position)
- wait_for_topic shell command (it's not used)
- orb_group_count() (sensors: dynamic sensor addition)
All use-cases benefit from the changed semantics: they are really only
interested if there is a publisher, not another subscriber.
The lifetime of the string is guaranteed because we never delete
DeviceNode objects and the strings in question are already on the heap.
This frees roughly 2.2KB of RAM on Pixracer
The limits the maximum queue size to 255, which I think is ok for the
forseable future.
sizeof(uORB::DeviceNode) is reduces from 128 to 112 on NuttX, and with
~80 instances, this saves over 1KB of RAM.
