Lock gCoreMutex before loading dictionaries to avoid deadlocks
Add locking of ROOT::gCoreMutex before opening datafiles, which can autoload dictionaries and possible deadlock in tcmalloc see: https://its.cern.ch/jira/browse/ATR-20263
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Add locking of ROOT::gCoreMutex before opening datafiles, which can autoload dictionaries and possible deadlock in tcmalloc see: https://its.cern.ch/jira/browse/ATR-20263