ipmi plugin: Implemented IPMI LAN connection and plugin improved

* Added instances support
* Removed the sensor removal in case of reading errors
* Added the lock to do not request new readings until previous reading is complete
* Disabled SDRs local cache
* Added the check for sensors 'is readable' flag
* Added the check for sensor event reading type (if sensor is discrete or not)
* Added checks for sensor states 'scanning disabled' and 'initial update in progress'
* Changes in error reporting