the destructor will block for way too long if we keep holding the
polling mutex while performing a transcation with the SDM power meter.
when reading, we now release the lock. afterwards, i.e., in between
transactions, we check the stop flag so the task terminates in a timely
manner once asked to do so.
instead of reading the main config's powermeter struct(s), the
individual power meters now are instanciated using a copy of their
respective config. this allows to instanciate different power meters
with different configs. as a first step, this simplifies instanciating
power meters for test purposes.
it is important to separate the capabilities of each power meter
provider into their own class/source file, as the providers work
fundamentally different and their implementations must not be
intermangled, which made maintenance and improvements a nightmare
in the past.
