Better control of collision object cache

[aleeper]

As demonstrated to Ioan, updating a planning scene via planning scene diffs received over the wire can cause memory issues with large octomaps. This is due to the cache size and how often it decided to clear the cache of obsolete object references.

Suggest either (a) the ability to manually clear the cache, or (b) automatically clearing the cache when an octomap is removed (and replaced by a new one).