Add expiration_period arg to TangoAttribute.read()

There is no way different then modifying the polling period to make the attribute readout return the value of the last event. Add expiration period concept to the attribute single readout.

Note that it might also have sense to add a global experation period for the attribute so one would not need to specify it at every readout but I think in some special occasion it still would make sense to pass it as argument to the read method. So, for the moment I just add this one since this is what we need in Sardana.