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)
Automatically ported from CMSSW_7_2_X #5136
maybe ```clusterSummaryCollection``` or better ```trackerClusterSummary``` or something like that. SummaryCollection is too generic for aliases and may end up colliding/overwriting someone else's settings.
Recall that these are useful realistically only in root fwlite scripts, Draw or Scan commands.