Cluster summary rewrite

The ClusterSummary and ClusterSummaryProducer classes were updated to improve performance, as documented here:

https://its.cern.ch/jira/browse/CMSTRACK-51
(https://indico.cern.ch/event/322710/session/5/contribution/2/material/slides/1.pdf)
is this pre-allocated set of 100 really needed?
The current config has only (up to) 8 positions filled.
it's easier to grow on demand here, since there's just one object per event of this kind.