Add counter for pending tasks to sensors. Wait until all tasks are finished before deconstructing anything

TODO: find a way to wait for all handlers dispatched by _strand to finish

The REST API should now be mature enough to be merged

Refactor: Move pluginVector_t typedef in own header file. Move restAPISettings struct to HttpsServer

Sensors which are part of an aggregating instance (e.g. in plugins PDU, IPMI, BACnet) now get a name prefix representing the agg. instance