PerfMon: enable thread-checker

Enable thread-checker for all PerfMon packages:

• PerfMonMTUtils: split capture method into a thread-safe and not thread-safe method
• Mark several legacy (non-MT) classes and functions as not thread-safe
• Delete unused ScopedMonitor
• Replace caching in PMonSD::CompDataStdSteps with unordered_map (hopefully fast enough)
• minor const and static fixes

cc @amete