Fix caching bug in ROOTClassEnhancer::IsA

When looking up the value of a cache in the map, it was possible
to temporarily have fLastClass and fLastType out of synch and cause
the program to return the wrong TClass.
Avoid holding gCINTMutex and gROOTMutex simultaneously in TPluginManager

By having a TObjArray be deleted outside of the gCINTMutex lock
we no longer have both mutexes being held at the same time and therefore
we avoid potential deadlocks.