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 (see also !274)

See merge request !273