iks: Params class code cleanup

+ create new ParamsUpdater class that is responsible for keep params
  values up to date
- remove subscribing for a moment because old way wasn't safe if
  multiple users use the same Vars class
iks: new better way to subscribe params

+ subscribe_params now return RAII object Subscription and when this
  object is deleted params are unsubscripted
+ this is safe for multiple connections to one Vars object
iks: don't fail in case of invalid configuration

+ catch szbase init exception and re-throw configuration exception