vdev: replace static list with an std::map
vdev: replace static list with an std::map

VDev::getDev() is used in px4_access, which is used in orb_exists. And if
the topic does not exist, it iterates over all 500 indexes, which is slow.
It was slow even if the topic existed, the map reduces runtime from linear
to logarithmic (there are around 80 items in the container).
This is only used on posix.

mje-nz/PX4-Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,dagar/Firmware,acfloria/Firmware,dagar/Firmware,PX4/Firmware,Aerotenna/Firmware,dagar/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,acfloria/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,PX4/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,Aerotenna/Firmware,PX4/Firmware,krbeverx/Firmware,jlecoeur/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,jlecoeur/Firmware,dagar/Firmware,dagar/Firmware,acfloria/Firmware