AlgExecStateSvc does not understand sub-slots

Problem demonstrated with a simple test in !979 (merged)

Algorithms running in sub-slots share a single execution state variable, because they exist in the same slot:

https://acode-browser.usatlas.bnl.gov/lxr/source/Gaudi/GaudiCoreSvc/src/AlgExecStateSvc/AlgExecStateSvc.cpp#0111

const AlgExecState& AlgExecStateSvc::algExecState( const Gaudi::StringKey& algName, const EventContext& ctx ) const {
   checkInit();
 
   auto& algState = m_algStates.at( ctx.slot() );
   auto  itr      = algState.find( algName );
   if ( UNLIKELY( itr == algState.end() ) ) {
     throw GaudiException{"cannot find Alg " + algName.str() + " in AlgStateMap", name(), StatusCode::FAILURE};
   }
   return itr->second;
 }

There are a number of possible fixes in the scheduler that could address the hang that this causes sometimes. However, the underlying issue seems to be with AlgExecStateSvc.

I propose to make it track execution states in sub slots where needed.