Stronger binding to spatial relationships

[SteveMacenski]

Take a ques from autonomous driving SLAM and maintain both a time-out and distance so that we don’t forget things nearby and out of frustum like their rolling SLAM implementations that only really track the local 100m window of road. Still some larger decay in the local area window of non-viewed items but higher. Similar to the frustum freezing work in another branch

[SteveMacenski]

v2 with a quasi-decay:

• do ordinary raycasting, properly using the signed distance field optimizations
• cache the measurements with the message. When time/distance are up, delete all measurements from a message rather than each individual cell
• each cell needs a reference counter of callers so it only decays with all measurements are gone.
• look at connected components in the multiresolution query to group together "substantial" blobs
• further accelerate readings in mini-blobs or orphans