[scheduler][callgraph] use counter instead of set of scheduled procs
[scheduler][callgraph] use counter instead of set of scheduled procs

Summary: No reason to use a set when an integer will suffice.  This further reduces GC churn.

Reviewed By: fgasperij

Differential Revision: D19888300

fbshipit-source-id: 9fc8c73f5

facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer