Retrieve record from SDR cache only once on first read() for each sensor and close the cache file afterwards to reduce the number of open file handles

Fix setting of MQTT topics for sensor templates, allow for using a template name as sensor name instead of the "default" parameter

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