Move the helpers for finding MT thinning decisions from DataModelAthenaPool to AthenaKernel.
As a result, I was seeing again bad_any_cast exceptions, because an internal symbol used for any_cast was resolved to different addresses in different libraries, causing any_cast to spuriously fail. Try to deal with this by introducing a set of out-of-line helper functions for accessing the extended event context that we use in preference to directly using the templated methods of EventContext.