This solves two problems:
1. redundant trackers making redundant requests
2. trackers being obliterated as soon as the primary metric component
is destroyed
It introduces a new problem where visiting more and more node and
allocation pages adds to an ever-growing list of trackers that can
assume lots of memory, but it solves the problem by using a
least-recently-used cache to limit the number of trackers tracked.
Michael Lange