IssueTracker

Currently encountering a movable group results in several unnecessary allocations and work that might be worth avoiding. A few things I found in my investigation:

- Pending.getNext allocates an iterator for the multiMap.pop() call.
- keyMap could avoid using LinkedHashSet and instead use ScatterMap/ScatterSet potentially.
- avoid use of `lazy` for `keyMap`
- groupInfos could use ScatterMap to great benefit since we are currently iterating through the values frequently which allocates and extra object and is also slower than scattermap's iteration