add sysExecute(EventContext) and extract errorCount from Algorithm base class

We need to extract the m_errorCount variable from the Algorithm base class, as it's not important on a per-instance basis, but rather on a job level. This is moved to the AlgExecStateSvc.

This merge request also adds an explicit IAlgorithm::sysExecute(const EventContext&), which should become the standard entry point to Algorithm::execute. This sets the local m_event_context variable, so derived classes can get quick access to the current context instead of going through the slower thead local Gaudi::Hive::currentContext().

Note that IAlgorithm::sysExecute() has been removed, to avoid "overloaded virtual" compiler errors, and should be replaced in all clients with IAlgorithm::sysExecute(EventContext), or if the context is not immediately available, with IAlgorithm::sysExecute(Gaudi::Hive::currentContext()).

All this is being done in preparation to re-introduce the ReEntrantAlgorithm class from merge !177 (closed) (see also !274 (closed))