loop through SLAM features about open_vins HOT 1 CLOSED

SLAM features will never be in the max track unless we have skipped updating for a whole window, which we never do. Using the cleanup() function after update removes the SLAM features from the tracking database after update, so at the next timestep only the newest frame track will be there (so the number of measurements is of size one). Thus the SLAM feature will never be in the maxtracks vector, so we need to ask the database if it has a measurement that has tracked into this new frame.

from open_vins.