Handle too short IPMI raw reads, move check for too long raw data extraction length to IPMIConfigurator::readSensor()

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

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

2 bugfixes (IPMI): take return value of ipmi_sensor_read into account; close sdrCache + ipmi-connection after reading