Need to use CINT mutex in Cintex::Callback::operator()

During the call the Cintex::Callback::operator(), several CINT
data structures are modified and therefore we must take the lock.
Minimize the time spent holding the Cintex mutex lock

To avoid lock order problems, we minimize the time spend holding
the Cintex lock by ROOTClassEnhancer. In addition, there was
a race condition in the previous code during the returning of the
value. That race condition is now fixed.