- RootHistogramSink: avoid having a 'registry' for each instance,
  instead keep one shared constant global in an anonymous namespace
  so it is hidden from the linker
- JSONSink: use the fact that `ofstream` closes the corresponding file
  when going out of scope
- BaseSink: use `std::set` with dedicted comparison operator instead
  of `std::map` which saves a bit of memory
- MonitoringHub: avoid one level of indirection in the type erasure
  which will type-erased function arguments to be passed in registers
  instead of forcing them to be passed through main memory
- MonitoringHub: `m_typeIndex` is an invariant once constructed, and
  does not need a type-erased function to be determined 'on the fly'