TrigDecisionTool: thread-safety fixes

A first set of changes to make TrigDecisionTool pass the thread-checker:

• Make the StoreGateSvc::keys method const (cc @ssnyder)
• Use asg::EventStoreType instead of custom typedefs for event store pointers and also make them const.
• Remove the DataHandle inheritance of the various DecisionObjectHandle classes and remove the internal caching of the decision object as the ReadHandle(Key) already does its own caching.
• Add thread-checker annotations for the Logger class.

cc @tamartin @jburr