Streamline MonitoringHub and Sink implementations

• RootHistogramSink: avoid having a 'registry' for each instance, instead keep one shared constant global one 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 should save a bit of memory
• MonitoringHub: avoid one level of indirection in the type erasure which will allow the arguments to the type-erased functions 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'